He's been voted the best Doctor Who ever, but David Tennant's rule as the Timelord is coming to an end. So how will he cope with life outside the Tardis? Johnny Davis, who has spent the past year trailing him, talks to Britain's most popular actor

David Tennant. Photograph: Ellis Parrinder

Last month David Tennant sold off his bed. It was, he admitted, "not the most delicious piece of furniture". It sat in reception at London's Absolute Radio looking every one of its 15 years in age, its wonky wrought-iron headboard accessorised by a Dalek bedspread and a handwritten sign: "Do not sit on this: prone to collapse".

Tennant was hosting Absolute's Breakfast Show alongside regular presenter Christian O'Connell. By 10am he'd played ping pong in the back of a Ford Galaxy, answered a series of questions from 12-year-olds and encouraged the actor Anthony Head to call in and sing "Lean on Me". Then there was his bed, being auctioned off for Children in Need. Fiona from Tadworth had pushed the bidding to £2,001, but off air O'Connell had a confession to make. The previous night he'd hosted a corporate do for the show's sponsors, British Gas. Things had got a bit carried away and everyone had climbed on the bed for a photo. "And it just sort of went 'poot'," O'Connell explained. "It was 10 of them. They were all trashed."

"There's never been more than two people on that bed," said Tennant.

Now its slats were snapped, the frame buckled beyond repair. "It's not the kind of thing you can just bend back into shape," noted Richie, the show's producer. "I felt bad; I told them it was for Children in Need," O'Connell said. "But you've seen the state of that bed – it's got 'Prone to collapse' all over it!"

"It wasn't prone to collapse," Tennant said.

On air O'Connell came clean, and someone from British Gas called in to do the honourable thing: take the now-useless bed off their hands for £5,000. "How's your head this morning?" Tennant asked.

Four hours of breakfast DJing behind him, he signed off with the Proclaimers' "King of the Road" and went outside to sign autographs and accept gifts from fans. Some had been waiting in the rain since 3.30am.

"That's quite good," he said, unwrapping one in the car that sped him towards his next appointment, at Radio 1. It was a Housemartins T-shirt, one of his favourite bands. "I bet it's extra large – they always think I'm big. And I'm only little."

Tennant was spending the day promoting his final three episodes of Doctor Who, the culmination of which will see him "regenerate" into a new Doctor, played by Matt Smith. After a chat with Radio 1's Fearne Cotton, there was a round of interviews with the TV listings magazines. Tennant asked his publicist which journalist would be attending from one particular title. "Hmm," he said. "She'll always go for the 'Who-are-you-shagging?' type question."

At Radio 1, he bumped into Chris Moyles. "So handsome," Moyles said to him by way of a greeting. Tennant explained he'd come from hosting a rival station's breakfast show. "According to the papers I seem to be leaving every week," said Moyles. "So you might as well have mine."

Fearne Cotton appeared. "We'll get you in just after the news; some questions from listeners – nothing bizarre." Tennant explained he'd been doing the promotional rounds. "Do you ever get tempted to make stuff up?" Cotton asked.

"So tempting," he said. "'Have you given Matt Smith any advice?' That's all I get asked. What am I supposed to say?"

"That's ridiculous," said Cotton. She consulted her notes. "Cross that one off."

Tennant wondered about the listener questions. "Are there rude ones? Do you get sent rude pictures?"

"All the time," said producer Stuey. "A lot of penises."

"Especially if you ask for something specific," said Cotton. "We did this thing asking people to send in pictures of their teddy bears – 50% were cocks. You get willies and boobs all the time."

On air Cotton asked Tennant about a poll that had voted him Britain's Sexiest Man, above Daniel Craig and Ewan McGregor, but also Jeremy Paxman ("Well, that's taken the sheen off"), discussed manual vs electric toothbrushes (Tennant's an electric man) and asked how his "complete army of fans" would cope when he's no longer on Doctor Who. "You know what will happen? Everyone will go: 'Oh, it'll never be the same.' And then two weeks in [to the new series] they'll go: 'Matt Smith: he's brilliant.'

"That's what happened when I was a kid, when Tom Baker left," he said. "That's just how it works."

It's possible, of course. But even Matt Smith must figure Matt Smith's got his work cut out. Though it was Christopher Eccleston who jump-started Doctor Who's regeneration from 1970s wobbly setted laughing stock to one of the BBC's biggest properties, a brand now reckoned to be worth £100m, it was surely David Tennant who sealed the deal. Not only has the role seen him surpass even the immortal Tom Baker as "The Best Doctor Ever", as voted by readers of Doctor Who Magazine, and there's no sterner jury, it's seen him become one of our most respected, most loved actors.

"David is arguably the most popular actor in England," says Patrick Stewart, who appeared with him earlier this year in the Royal Shakespeare Company's Hamlet, the film of which is on BBC2 this Christmas. "There was more anticipation for that production and David's performance than anything I've ever been in."

Famously, it was Doctor Who that made the three-year-old Tennant want to act. At school he'd carry around a Tom Baker doll (though he was too shy to ask his parents for Baker's assistant, Romana). As a teenager he wrote Who-themed essays called things like "Intergalactic Overdose", as his English teacher, Mrs Robertson, helpfully showed the News of the World recently. Even when he got the role, he lobbied the producers to change the credits to correct a longstanding inconsistency that had always bugged him – everyone knows the lead character is called "The Doctor", never "Doctor Who". (One afternoon I recalled how Jon Pertwee's Doctor used to dispatch foes with a neat line in kung fu. "Actually I think you'll find it was Venusian aikido," he corrected, not entirely humorously.) While all of this might have made him ideally suited to the job, leaving it has traditionally proved rather harder. None of the other actors who've played the Timelord have ever really lived it down. Baker has confessed that everything since has been "a muddle and a disappointment, an outrageous failure", and fear of typecasting led Eccleston to crash back to earth after just one series. It was a problem not lost on Tennant – or his agent, who suggested that even a bit part on the show would mean "I'll never work again."

"It did take me a few weeks to think it through," says Tennant, 38. "But the only other option is you don't do the job. I remember waking up one morning thinking: 'I can't turn this down. Even if it's the wrong thing to do.'"

Yet his acting credentials already put him in a different league to his predecessors. Olivier Award-nominated at 31 and a veteran of the RSC, he has managed to fill the three remaining months of the year when he's not been in Cardiff filming Doctor Who with an impressively wide range of boldface gigs: the lead in Hamlet and Berowne in Love's Labour's Lost running on stage concurrently, a Harry Potter film and several weighty TV dramas, including playing Sir Arthur Stanley Eddington in Einstein and Eddington. As well as Doctor Who and Hamlet this Christmas, there's the Stephen Poliakoff film Glorious 39 and the role of dastardly Lord Pomfrey in St Trinian's 2: The Legend of Fritton's Gold. Which certainly shows range. "It's all the same thing," Tennant smiles. "It's all acting. I think Shakespeare was a man of the people."

"When I started out, if you got known for one role, forget it," says David Morrissey, who co-starred with Tennant in Doctor Who and the 2004 TV musical-drama Blackpool. "But David's Doctor won't be the millstone around his neck that it's been for actors in the past. It might weigh him down in a personal way – walking down the street and stuff – but he's so gifted it won't ever restrict him professionally."

What's more, Tennant's popularity is now such that he occupies a fairly unique position among his peers. He is as likely to give an interview to the University of Cambridge's Shakespeare journal on Mark Rylance's 1989 production of Hamlet for the RSC as he is to appear on Top Gear's "Star in a Reasonably Priced Car", or turn on the Blackpool Christmas lights. In February he presented Comic Relief with Davina McCall – a remarkable thing for an actor to be asked to do. "Yes, but that's to do with Doctor Who," Tennant says. "I don't imagine I'll be in the frame for things like that any more. I'm sure in two years' time they'll want Matt Smith to do Comic Relief. I suspect I'm just passing through, really."

David Tennant. Photograph: Ellis Parrinder

Perhaps. When he joined Doctor Who in 2005 it made the BBC News At Six. It may be no exaggeration to say his departure is a national event. "David is very sad to leave," says his friend, the actress Arabella Weir. "But when do you leave the party? When everybody has stopped asking you to dance and is going: 'Look at that sad old cow, he's still here'? You don't know, is the short answer. You just have to make that judgement." Tennant's final episodes will be broadcast on Christmas Day and New Year's Day. Because filming happens non-sequentially, the last scene he recorded as The Doctor has actually already aired – an episode of spin-off The Sarah Jane Adventures, which went out last month. His final words were an unprepossessing: "You two, with me, spit spot." "It couldn't have been less memorable or less significant," he says. "It was robbed of any epic quality, but that was probably best. There are a lot of scenes in the final story that are very sad, and were very sad to play. If one of them had coincided with the actual final day, I'd have been a puddle."

Come 1 January, writer and executive producer Russell T Davies is counting on us feeling the same way – greeting the New Year in cheery fashion, watching Tennant expire at the hands of The Master. "I can't watch it without crying, literally," says Davies. "I was checking it for the music cues the other night, which must have been the 17th time I've watched it, and I ended up crying. It's heartbreaking."

BBC Wales makes Doctor Who in several large hangars in Upper Boat studios near Pontypridd – Torchwood and The Sarah Jane Adventures are also filmed here. One stage houses a vast permanent set of the Tardis interior, and round the back there's an endless props area, an I-spy of half-exploded Daleks, killer Christmas trees, Ood heads. A fortnight before filming each episode begins, the cast and crew meet in a nondescript Cardiff hotel for a script read-through. There is some secrecy surrounding these meetings, for reasons best illustrated by the time they had to eject a journalist from the Sun who'd been discovered sitting among them, mid-read-through.

In early February I watched the cast read "The Waters of Mars", an episode transmitted in November, arranged around a long table with Tennant, boyish in a Dennis the Menace-style jumper. Two weeks later the script was being realised in three dimensions. Set aboard a Nasa-style base, it required Tony Award-winning actress Lindsay Duncan – last seen playing Margaret Thatcher in TV drama Margaret, today playing space-suited ball breaker Captain Adelaide Brooke – to be thrown around amid various explosions. "I can't roll on the floor because of the gun," she worried to director Graeme Harper.

"Dare I say it?" wondered Harper. "But are your knickers going to be OK?"

"I'm wearing an all-in-one," she advised.

Tennant was in the canteen ("The Blue Box"), not required for filming until after lunch. "You do get slightly institutionalised here," he said. "For four years I've always been going back to Cardiff at some point in the near future, so when I leave it will be like leaving campus. I don't mean to get things out of proportion, but I was keenly watching George Bush leaving the White House, and the thought of how his life is going to change… I'm not saying his life is like mine. I'm not the leader of the Free World, I'm really not…"

Which would make Matt Smith Obama, of course.

"Oh, that's not really worked out very well for me, has it? It's just the thought that you hand over… and it stops. Maybe I'll be whisked up into something equally all-consuming."

One thing he may adjust to more quickly is a reduction in his own visibility. "We always thought when the honeymoon period was over it would settle down, but with every series it seems to get more attention. The viewing figures went up last year considerably. It's sort of bewildering." While Tennant fully appreciated the level of attention Doctor Who would bring him personally, it's not necessarily something he regards as a perk. He was in the role for a matter of weeks before a tabloid reporter had him out of bed at 7am, threatening to run a story involving a brothel, prostitutes and drugs. "Funnily enough, they didn't have photographs." It's not what he joined Equity for.

"You know you're going to have to cope with it on some level, but until it happens to you I defy anyone to really know what it feels like," he says. "When I saw people who were famous, and people whispered and pointed, it felt as though a very powerful individual had walked by. And actually, once you are that person, it just feels scary. All the time."

He says he was helped enormously by having Billie Piper with him for his first year, playing the Doctor's companion Rose Tyler. "She'd been through it for years. And she had it much worse – women tend to. She had become such a great friend and a real help through the madness that was beginning to explode. And then losing her, and thinking: 'I'm on my own!'"

If Doctor Who saw Tennant join the select group of males favoured by the gossip pages, unlike, say, James Corden or Russell Brand, he's done a remarkable job of keeping his personal life just that. He's adept at giving nothing of himself away while remaining a charismatic personality.

He apparently dated Sophia Myles, who played Madame de Pompadour in the show, and has been linked to his assistant director and another BBC Wales staffer. It's likely he's currently seeing Georgia Moffett, who played the Doctor's daughter in one episode and is ex-Doctor Peter Davison's daughter in real life (at which point you may think he's taken his enthusiasm for Doctor Who as far as it can go – "It can be odd when David comes round for Sunday lunch and we all sit at the table; me, an ex-Doctor, with my wife, and David, another Doctor, and my daughter," Peter Davison revealed). And years ago he went out with Anne-Marie Duff, now married to James McAvoy. But you won't hear that from him. "Relationships are hard enough with the people you're having them with," he says, "let alone talking about them in public."

"I resisted jumping his bones," says Billie Piper, "but women really fancy him. He's got a gorgeous face, and an energy that's contagious – the spirit of a child. My girlfriends were all in love with him." One female critic described his Doctor as "the first Timephwoard". His favoured trick for dealing with the inquisitor who inches towards the aforementioned "Who-are-you-shagging?" type question is a kind of reproachful look. "He's avoided any scandal because he keeps shtoom," says Piper. "He very rarely talks about anything that isn't related to his career or acting. You never see him falling out of clubs. He's never off his face. He's got far more patience than I have," Piper adds. "I don't mind signing autographs, but it becomes the topic of conversation at every social event you go to. It starts off: 'So how are you?' Then it's: 'Anyway, about Doctor Who…' It's at that point I start reaching for the wine."

In April, Tennant was at BBC Television Centre to promote the first of 2009's Doctor Who specials, "Planet of the Dead", to air that evening. It was 8am on Easter Saturday morning, yet BBC reception was uncharacteristically busy. Specifically, it was uncharacteristically busy with children. Tennant was due on BBC Breakfast and arrived cheerful as ever, wearing a jacket and thin tie. "You're on after the Association of British Drivers," said Kate, Breakfast's producer. "The people who blow up speed cameras."

"I didn't know there was such a thing," Tennant said.

Julia, the editor of programmes, appeared with five children. "It's my day off," she explained. "But I came in especially."

Tennant signed autographs for everybody and posed for photos. He was ushered into the green room. "This is Frederick," said Kate. "He's reviewing the papers just before you."

"Can I be the first to ask," Frederick said. "Would you mind signing this for my sister? She's desperate to have your autograph."

Then Maxine ap-peared. "I'm one of the holiday newsreaders," she said. "Would you – I mean, you're probably fed up of doing this – would you sign this for my nephew?"

Tennant went on air and was interviewed by presenters Sonia Deol and Charlie Stayt. I watched from the control room. Stayt suggested that while the previous Doctors had been "interesting, quirky characters", Tennant was the first to be a sex symbol. "Lots of snogging you've done," he said.

"Not lots," countered Tennant. "More than Jon Pertwee did."

He was asked what he found scary in real life ("I'm not a fan of a rodent"), about changing his birth name from McDonald to Tennant for Equity by picking Neil Tennant's name from Smash Hits ("I could have been David Kajagoogoo") and whether he is ever able to go out in public and "be normal".

"Has he talked about the next Doctor?" asked someone in the control room.

"No. Can we ask him about the next Doctor?"

They went on Wikipedia. "It's Matt Smith."

"Matt Smith," it was relayed to Deol's ear. "The new Doctor. Very young."

"What about the new guy?" she said on air. "What advice have you got for him?"

"I don't think you can give anyone advice about stuff like that, can you?" said Tennant.

Afterwards he was collected by producer Kate. She was holding a pile of paper. "I shouldn't have walked through the newsroom," she said. "More requests."

"More requests? We're not going to be allowed to leave, are we?" said Tennant, not unkindly.

Kate seemed to be chewing something over. "I don't care. I've lost all dignity," she said to me. "I'm going to ask for a photo."

The smell that reminds David Tennant of childhood is his father's homemade chicken and leek soup. He grew up in Paisley, near Glasgow, the youngest of three. His dad, Sandy, was a minister and later moderator of the General Assembly of the Church of Scotland. His mum, Helen, devoted her life to charity work and helped found Paisley's Accord hospice. She died of cancer in 2007, aged 67. There's a gap of six and eight years between him and his siblings, Blair, who works in the music business, and Karen, a nurse and teacher. His upbringing was grounding. "Not all men of the church are necessarily good human beings, but my dad happens to be. My mum was, too," he says. "I feel very thankful for that."

He gained a place at the Royal Scottish Academy of Music and Drama at 17 – their youngest pupil. It was a combination of this and moving to Glasgow with his sister that brought him out of his shell. "Leaving home was one of the best things that happened to me. I was a bit green. I wasn't a particularly worldly 17-year-old." At drama school he was "surrounded by all these exotic older people who seemed to know about life. So it was a really brilliant time." He acted Ken Stott off the screen in Takin' Over the Asylum, the 1994 TV mini-series set in a Glasgow psychiatric-hospital radio station, playing bipolar DJ Campbell ("This is for all you having ECT tomorrow: hope you get some 'Good Vibrations'!"). On set he met Arabella Weir, moving to London the following year to spend five years as her lodger. He complained she never put the heating on; she teased him about alphabetising his CDs. Tennant was soon being talked up as a rising star of theatre, notably for comic roles – Touchstone in As You Like It, Captain Jack Absolute in Sheridan's The Rivals. "Even aged 22 he had an unusually strong sense of self," says Weir. "Most actors are in the business of wanting people to like them. He was: 'This is what I can do; I'm not interested in doing other jobs.'"

He was mesmeric as love-struck policeman DI Peter Carlisle in 2004's Blackpool and head- turning as Russell T Davies's Casanova a year later. "No one could get it right," says Davies. "Everyone was playing the swarthy romantic lead – and here was this man who simply danced all over the script."

"I think I've just been lucky, really," Tennant says, "because I'm not conventional leading-man stuff. I'm slightly left of centre. I remember going up for Casanova thinking: 'I haven't got a chance – the other people are much more traditional square-jawed types.'"

Plus, everyone agrees he's a generous team player. "When you're playing the leading role in a play, you have responsibilities that go beyond saying the lines," says Patrick Stewart. "You lead the company; you set an example. The stress of Hamlet must have got to him, but it never seemed to. You'd see him in the wings beforehand and you would have thought he was preparing to go out for dinner, he was so relaxed."

"Everyone said I would adore working with David, and they were right," says Kylie Minogue, one-time Doctor Who companion Astrid Peth. "He made me feel at ease. I also felt he trusted me, which was important – it was a step back into acting for me. My time on Doctor Who was hard work, but I felt somehow I was 'home'."

Tennant was back at Upper Boat in May, filming his final two Doctor Who episodes, "The End of Time". "Three weeks to go now," he said. "Three weeks and counting." On set John Simm was doing something terrible as The Master that it would be wrong to reveal. "What have you done, you monster?" shouted Bernard Cribbins, who's returning as Wilfred Mott, father of Catherine Tate's Donna Noble. Tennant was feeling good about his final scenes. "It's all very heroic," he explained. "My final 100-yard dash." They were being even more wary than usual about leaks. Some on-location photographs had appeared that week, to everyone's disappointment. "And someone was discovered here the other day with a scanner," Tennant tutted. "They had tuned themselves into the radio mics inside the building and were writing down the dialogue."

In June, the month after filming their finale, Tennant, Davies and John Barrowman travelled out to Comic-Con, the annual "popular arts" convention in San Diego. (Doctor Who has a US fanbase, while Torchwood has become the top-rated show on BBC America. Davies now works for BBC America in LA.) While he was there, Tennant found himself an American agent and did some auditioning. "Just sniffing around, vaguely seeing what was out there." This resulted in him being cast as the lead in comedy-drama Rex is Not Your Lawyer, a role NBC had been trying to fill for months. Tennant will play Rex Alexander, a panic attack-prone Chicago litigator who starts coaching his clients to represent themselves. The pilot's being directed by David Semel, who did House and Lost. "I went to bed one night having had conversations that we could come to terms for this pilot, woke up, and it was on the front of the Hollywood Reporter," he says. "It's a different world in America."

"I'm sure Hugh Laurie's success with House is an appropriate comparison," says Catherine Tate. "David's a brilliant comic actor. America would be mad not to love him."

He's also likely to play opposite Simon Pegg in John Landis's black-comedy remake of Burke & Hare, about the 19th-century body snatchers, and the internet is convinced he'll be the Riddler in the next Batman. "I probably should be," he says. "But you'd think my agent would have mentioned something if it was true."

Tennant finished his chat with Fearne Cotton, remembering to plug the upcoming episode. That evening he was off to Stratford to see his friend Richard Wilson in Twelfth Night. In 10 days' time, visa permitting, he'd be filming his pilot in LA. But first it was off to Television Centre and Simon Mayo's Radio 5 show. Down one corridor he ran into a class of schoolchildren being given a guided tour. They couldn't have been more stunned if Tennant had stepped out of their own TVs. "And they've just seen the Tardis outside," their teacher beamed – a replica prop lit up outside reception.

"That's how I got here," mugged Tennant. "I've just arrived."

A lady from BBC promotions appeared. "If you don't mind, I've got a 16-year-old niece in Australia. She loves three men: you, some Australian footballer and Roger Federer."

"What an interesting combination," said Tennant.

"So if you could just sign…"

On Mayo's show they discussed the upcoming Glorious 39 and St Trinian's 2 with film reviewer Mark Kermode. "Did you like St Trinian's 1?" Tennant teased. "It was one of the worst things that's ever happened to me," said Kermode.

But what everyone really wanted to talk about was Doctor Who. Tennant explained he'd just watched his final episode, with some key crew (more tears). Beforehand he'd been nervous. Afterwards he realised they'd done what they'd come to do. They were handing it over in rude health.

"I feel like I've done all right by my eight-year-old self," he said.★

Doctor Who will be shown on Christmas Day and New Year's Day on BBC 1 at 6pm