"The anxiety is unbearable," said Oscar Wilde, one of history's great should-have-been Doctors. "I can only hope it lasts for ever." Such is the case for lovers of Doctor Who. For months they've both avoided and vacuumed up any scrap of information about this Saturday's 50th anniversary special The Day of the Doctor, torn between curiosity and the self-denying desire to see the show play out as intended. You want to know, and you don't want to know.

The special could well be BBC Drama's biggest event ever, with a worldwide simulcast, 3D cinema screenings and a security blackout. It features current incumbent Matt Smith, his pin-up predecessor David Tennant, Billie Piper as someone who may or may not be Rose Tyler, and a hitherto unseen incarnation of the Doctor possessed by certain dark secrets and played, in testament to the reborn show's stature, by John Hurt.

Does Doctor Who's showrunner feel the weight of the occasion? Are we about to witness Steven Moffat and The Burden Of the 50th?

"It's such a hell of a thing to work on, and there is a sense of responsibility," admits Moffat, the 52-year-old from Paisley, Scotland, who took charge of the show with the end of Russell T Davies's tenure in 2009. We're talking in a multi-coloured circular thinkpod in the centre of New Broadcasting House that looks like a Fisher-Price Tardis control room. "In the end I thought, let's just try to make it a really, really good one. Do what James Bond did with their 50th – a story that's so good in its own right that it stands up as a 50th special."

Feeling the pull of tradition, some fans had wanted to see all 11 Doctors somehow reunited, or at least the post-revival trio of Christopher Eccleston, Tennant and Smith. Such poly-Doctoral plans were scuppered when Eccleston declined to appear. Moffat had met him for a "very amiable and gentlemanly" conversation and the actor considered it "quite seriously" before saying no. "It's just not the sort of thing he does," concludes Moffat. "The ninth Doctor turns up for the battle but not the party. But Chris was perfectly sweet and kind about it. And contrary to what was written at the time he in no way messed us around."

Instead the birthday story will concern a particularly important day in the Doctor's life. "It's a turning point," says Moffat, trying to explain without giving anything away. "We don't often do good character episodes for the Doctor. He's usually the one who catalyses other people's big emotional moments. We never see him when he's alone, he's always with his human friends.

"But if he meets another Doctor, what would he say to himself? What does he ask himself? This one is about him, for once. Am I making it sound too heavy? It does have its darker moments but it is a romp too. It's fun and it's funny. And the trailer doesn't exaggerate. It is big."

Then again, given that the BBC were able to keep entirely quiet the surprise reappearance of underappreciated eighth Doctor Paul McGann in last week's stirring online prequel The Night Of The Doctor, it's entirely possible that Moffat is lying through his teeth and we'll be knee-deep in Doctors on Saturday. I wouldn't mind if he was.

"Steven still has the qualities of a brilliant teacher," says Mark Gatiss, Who scriptwriter and co-creator with Moffat of Sherlock. "As he'd freely admit he's got a streak of Scots grump to him, but he's brilliant at generating enthusiasm for your ideas. He's got a fantastic story mind and he's always interested in pushing a script in a different way – not perversely, but he flips it. You think you knew where the story should go, but he'll get you to think about it totally differently."

The Doctor Who that Moffat took over from Davies in 2009 was the most successful revival in British television history. Its mix of escapism and family-friendly emotional warmth showed Britain certain sides of itself that had been hidden during the realist, mad-for-it 90s: imaginative, fantasist, psychedelic, a little bit daft but wedded to the notion of doing the right thing. Being a Doctor Who fan turned out to be like being in the French resistance – as soon as the show came back it turned out that everyone had been one. As the star writer who gave Davies' The Empty Child (boy with gasmask face terrorises Blitz-era London) and quantum-locked living statues the Weeping Angels (the show's first truly A-list recurring monsters since the Sontarans), Moffat was expected to take the series in a more explicitly terrifying direction as showrunner.

Instead his Who became more wildly free-ranging and more labyrinthine in its plotting. Many of the episodes were among the best of Who, especially in their understanding of childhood and the strange little girl Amy who meets the Doctor and then discovers in adulthood that her imaginary friend was real. Much of Moffat's own writing, from Matt Smith's journey into a maze filled with Weeping Angels to last season's ludicrously intense finale The Name of the Doctor, was among the best the show has seen. Some viewers, however, saw in this new complexity Doctor Who disappearing up its own space-time vortex.

"It's funny, everyone thought it was too complicated for someone else, not them," says Moffat. "I don't want to be mean, but eight-year-olds seem to have no problem with it. Doctor Who is unashamedly a clever show. There have been calls for us to dumb it down but we just don't. We're dealing with children who can read long, complicated books while tweeting and playing computer games all at the same time. You've got to be ahead of them."

Sometimes it feels like Moffat is caught between the hardcore fans – vocal, possessive, perhaps too forensic in their love of what is only a TV programme – and a much larger corpus of general viewers who just want a good old thrill-ride laced with some unknowable cosmic terror of a Saturday night. Or maybe there's a fan in every mainstream viewer and vice versa. The fanboy and the fangirl are no longer marginal figures, but are driving the culture.

"I love Doctor Who fans," he says, "and I am a Doctor Who fan, but the show is not targeted at them. And to be fair most of them say: 'For God's sake don't make it for us.' They want it to be successful. They don't want it to be a niche thing, because then it would die."

Moffat's earliest Who memory is of watching Patrick Troughton and wondering where the real Doctor, William Hartnell, had gone. "That's Doctor Who now," his father told him. Young Moffat, then perhaps five years old, thought he was far too young for the role. "Oh, the irony," he says. "I was the original angry fan." The Doctor Who of the 1960s cemented Moffat's idea of perfect televisual fear. "It was terrifying," he says. "It wasn't the camp or sweet or nice thing it became for a while afterwards. It wasn't improving or good for you, it just wanted to scare the crap out of you. It was the bad boy of children's television."

During the dead years between cancellation and revival (1989-2005) Moffat moved in a circle of diehard fans based around the Fitzroy Tavern in London, which included working TV professionals like himself and Davies, writers of fan fiction and straightforward Who lovers. In those days the programme seemed destined to fall into the same category as The Prisoner – fondly remembered but never coming back.

"Were we keepers of the flame," Moffat wonders, "or just moths circling that flame, deluding ourselves that we were influencing the fire?" They would fantasise about what they'd do if Who ever did return. As a TV writer with a few well-known shows to his name – Press Gang, Chalk and then Coupling – Moffat thought he might be in the running. He wrote a couple of short stories and an affectionate Comic Relief sketch The Curse Of The Fatal Death. "That form of fandom was much more active than it is now," he recalls, "because you only had what you could create yourself. There was no Who on TV. We had nothing."

The Doctor and the Doctor join forces for the 50th anniversary edition. Photograph: BBC

But the show was relaunched, and he did begin writing for it. Creating vampiric statues and clockwork robots under the bed and darkness that ate human flesh, Moffat was rapidly tagged as Doctor Who's own Captain Frightening. "The king of terror thing was something that Russell said about me," he says, "but people forget that The Girl In the Fireplace is Mills & Boon Doctor Who. The monsters, outrageously, are offed 10 minutes before the end and there's no jeopardy at all."

Davies's Who had been brash and populist, with a common touch centred on Rose Tyler's working-class family and David Tennant's emotional availability. I ask Moffat how his own more circuitous version begin to gestate. "I don't really think of it as 'my version'," he says. "I just think of it as making Doctor Who. The great joy of the show is that it can be anything you want at any time. A fairytale one week, then a horror story the next and a romcom the week after that. You don't start with a big vision – you start with the most exciting thing you can think of to put on TV on a Saturday night."

Now that Doctor Who matters again, the job of executive producer is more high-profile than it was the 1970s. People know who he is; fans call him the Grand Moff. And in the social media era every single facet of Who is analysed in painstaking detail on an internet that breeds strongly held and not always generous opinions. One is that Moffat's female characters are empty vessels defined only by their relationship to the Doctor: Amy the childhood friend, River Song the brave-faced but pining on-off wife, Clara Oswin the mystery to be solved.

"The thing is," Moffat argues, "the show is about the Doctor, and the effect he has on people's lives. We don't tend to see the companions away from him because if we did that it wouldn't be Doctor Who. I've heard this criticism of lack of interior character about River Song – but she's the only one who's ever turned him down. I think I have written companions who've carried on with their own lives. The Doctor is central not because I think men are better than women but because he's the central character. How is that not also true of Rory?"

Amy, he says, arose from the doubly complex challenge of introducing Matt Smith's new Doctor and a new companion at the same time. Usually there's a companion we trust, so that the new Doctor can prove himself to her – and by extension, to the audience. We need to know that he's still himself. Amy provided a different way in. "The question with Amy was, what would it do to you if you met your imaginary friend and then he didn't turn up for another 20 years? What if you had an imaginary friend who let you down? So you get feisty, difficult, trust-issues Amy. And that's how I start a lot of relationships."

The show is not just bigger on the inside now. It is bigger on the outside, the BBC's flagship property, seen by 77 million people in over 50 countries. Moffat has just spent his afternoon meeting the people from Brand Who. "The world of Doctor Who is now bigger than the TV show, obviously, but you can't ever stint in the show for the other stuff," he says. "My day consists of fighting to get enough writing time." He strongly dismisses the fear that as the show gets bigger it could lose touch with its British roots. "If you want to sell a show to the world," he says, "make it as British as you can. America likes Doctor Who because it's British. Do what it says on the tin. It would be insane to make Doctor Who less British."

A few weeks ago Moffat gathered all of the show's writers together so that they could tell the fully plotted and complete story of the eighth season of Doctor Who – Peter Capaldi's first, and the debut of the 12th (or is he the 13th?) Doctor – between them, from start to end, to see if it works. To see how one episode answers another, to see if the twists and reveals feel right, to find out if it says what it needs to say.

"I'm a big believer in oral tradition," he says. "You've got to get it into your head. There's nothing worse when you're writing than having to look at a pile of documents. You should bloody well know." He doesn't want to tell me what Capaldi is going to be like, and I don't want him to. I want him to show me when it's time.

I wonder what five-year-old Steven Moffat would make of his successor's version of Doctor Who. "I think he'd love my take on Doctor Who," says Moffat with a grin, "because it's his take."