DAVID Tennant arrives at the Glasgow hotel, smiling and relaxed.

He’s quite striking; as tall as the wildest tale he appeared in as Dr Who (he’s six feet one), and wearing his Broadchurch beard, tight hipster jeans and what looks like a very expensive jumper.

If you look closer at the knitwear however you realise it’s full of little holes. Whether the rips are designer-deliberate or not it’s hard to tell.

But as conversation takes hold and we discuss madness, schizophrenia, depression and skewed self-perception, the torn knitting is an apposite metaphor; we may look pattern-perfect from a distance – but come a little closer and the flaws are there to be seen.

Tennant is in town to talk about his latest film project, the hugely challenging Mad To Be Normal, in which he plays a very flawed character indeed.

He stars as controversial Scots psychiatrist and mental health pioneer RD Laing, the man from Govanhill who challenged public perceptions of schizophrenia, maintaining it was a result of poor mothering rather than a fault in genetic wiring. Laing argued that family unity, understanding and empathy could contain the condition.

Yet while the hugely intelligent and erudite Laing won hero-grams from the likes of the Beatles and Sean Connery for his radical work, which included rejecting the then conventional electric shock treatments, he treated people, including his family, badly.

He was often a drink-addled aggressive bully. While his first family (he married twice and had 10 children) were left in penury in Glasgow, Ronald Laing lived the playboy life. While his first wife had to pawn jewellery to feed her children, Laing played around with bunny girls and actresses and with the minds of the rich and famous, supplying LSD as a mind-expander.

So how did David Tennant view the notion of playing such a divisive character? Was this an even greater challenge than, for example, playing the pre-op transsexual Davina in a 1993 episode of sitcom Rab C Nesbitt? (Which he managed with real nuance.)

It certainly must have required a greater understanding of his subject than was demanded during his Sonic Screwdriver days as Doctor Who.

“Well, it’s nice to see you remember my Davina,” says Tennant, smiling.

“But the reality is I’ve been intrigued by the man for some time, ever since I saw a play about artist Mary Barnes, in which he was featured over there." (He nods in the direction of his alma mater, what was the former RSAMD, now known as the Royal Conservatoire of Scotland). "Plus, I knew he was tall and rangy and I really fancied having a crack at him.”

The actor adds: "I hadn’t joined the dots about him, but I wanted to explore him. I loved the idea of playing this strutting peacock, yet a man who was doing extraordinary work. From my point of view this was delicious.”

Mad To Be Normal focuses on the period in RD Laing’s life in 1965 when he set up Kingsley Hall, an East London treatment centre for the mentally ill.

Traditional psychiatry was sidelined and the lines between doctors and patients, who lived together, were deliberately blurred. Straitjackets were out and LSD was part of the treatment plan. Laing became a cause celebre and Kingsley Hall something of a giant goldfish bowl. And Tennant says no-one could tell who were the mad fish.

“That’s true. Apparently Van Morrison visited. [The Beatles and Sean Connery were also said to be followers of Laing.] It was an extraordinary world and it’s hard to get your head round this from a 21st-century perspective, that doctors and patients were living together.

“It’s fair to say the Kingsley Hall set-up had mixed results. Some patients today say he changed their lives for the good.”

Yet, a couple of RD Laing’s patients jumped off the roof, David.

“Yes,” says the actor, taking a breath and smiling. “It was, well, yes, controversial. But when you start to study this man you see so many paradoxes and contradictions. On the one hand he had wells of empathy, he could pour love into someone he had never met before yet could be something of a bully with his own family.”

Laing was a political philosopher – but he would punch people for no reason, treat lovers with disdain.

Did the actor who grew up in Ralston in Renfrewshire see a risk of alienating the audience, given this un-likeability? “I tried to be honest with him. I don’t think you can ask an audience to like you. And Laing was charismatic. People did fall for him. While he drove some people mad he also elicited reaction.

“I just took each moment as it arose and portrayed his qualities and faults.”

Tennant has played several tortured characters to great acclaim in recent times such as Richard II, and Hamlet. Has playing men with severe personality disorders taken him closer to an understanding of mental fragility, an awareness that behaviour and mood can change dramatically when we’re assaulted by (tragic) circumstances?

“Yes, you’re toying with the idea of your own mental fragility a lot of the time,” he admits. “You are trying to access moments where you can understand the truth of being in a very difficult situation, seeing how far you can push it, or with someone like Hamlet you are calling upon your recollection of grief.

“So you are trying to recreate how that felt. Or in Mad To Be Normal you are calling on your experiences as a parent, whether you are a good or a bad parent.”

Thankfully, Tennant grew up in a balanced, loving home. His father, Sandy McDonald, was a Church of Scotland minister and one-time moderator who co-presented STV's religious magazine programme, That's The Spirit, during the 1980s and later made a cameo appearance alongside his son on Doctor Who. (He died last year of pulmonary fibrosis, having called on senior church figures to support plans to give terminally ill patients the "right to die".)

But does the actor (the Tennant name-change was inspired by the Pet Shop Boys’ Neil Tennant) think Laing acknowledged the contradictions in his own behaviour? He fractured two families while arguing professionally the importance of the family unit.

“I think any human being justifies his own contradictions in the moment they are in them. Often the cleverest person in the room is often their own worst enemy because they can argue their way out of anything.”

He pauses before continuing. “I don’t think Laing hated himself. And he attributed his own problems to his upbringing, which is touched upon in the film.”

Ronald Laing said his mother insisted on bathing him until the age of 15. He claimed she had a voodoo doll of him which she stuck pins in.

Does Tennant believe Laing, perhaps as a result of his upbringing, suffered from mental illness? Did he (perhaps subconsciously) go on to study and redefine schizophrenia as being a maternal hang-up because he too suffered from it, and blamed his mother? “Well, he certainly acknowledged he suffered from depression. And I don’t think he was shy from the notion he was mentally fragile himself at times.

“He was certainly open to the notion we are all on a shaky nail and this empathy and understanding is what made him explore his ideas. It’s true to say he felt mental illness was closer to all of us than we care to admit.”

Has David Tennant ever been touched by mental illness such as depression?

“I don’t think I’ve been touched by serious depression. But I think it’s a sliding scale. I don’t think mentally fragility is binary. I think I can recognise moments in my own life where I’ve slid up and down that sliding scale.

“I did a BBC TV series way at the start of my career, Taking Over The Asylum [1994, by Donna Franceschild], which asked similar questions about how fragile we all are. And since then I’ve been more aware. And when I’ve heard of people slipping in and out of an episode [of depression] it’s certainly not something that has been alien to me.”

He thinks for a moment. “I’m not saying I would have been diagnosed with manic depression but I can understand the gateway in and out of this sort of episode. It’s true of depression and other forms of psychosis. Certainly, doing this film and reading a lot of Ronnie Laing’s work has confirmed to me we are all just a breath away from this kind of fragility – more so than perhaps we would care to admit.”

Are actors more likely to become depressed, given they often have to reach into the darkness? And does that other pressure on an actor’s emotional state – the fact they are continually waiting to be decided upon – increase the fragility, given the thespians' perpetual need to be loved?

“Absolutely,” he says, smiling. “But in being part of a profession that constantly rejects you, or puts you up for awards, well you have to step back and tell yourself it’s all pretend. You need to keep a healthy perspective.”

How does David Tennant, who knew he wanted to act from the age of three, keep his 45-year-old head in check, given the success of recent times? Yes, he has his family, actress wife Georgia Moffet and their four children, to remind him of normal life. But few actors have the chance to see-saw between London's West End (in the likes of Don Juan) or feature films and television.

“I think there is a real danger in actors talking about how difficult their life is,” he says in more serious voice. “Ultimately, it’s all make-believe. The job may be about leaning towards the precipice at times but you don’t actually tumble over.”

He breaks into a smile. “I love this job and I’m hugely committed to it. But I also realise that it’s all silly at the end of the day.”

Does having grown up in the west of Scotland (he was a Paisley Grammar pupil) keep him balanced?

“Oh, yes,” he says, grinning. "The ‘Who do you think you are?’ mantra echoes around your brain as soon as you get too big for your boots.”

The very fact Tennant agreed to make the RD Laing film suggests his footwear is still a good fit. Tennant certainly didn’t do it for money. The budget for the film was so small the director wouldn’t reveal it. And there was the waiting time. The film of the life of RD Laing (who has been the subject of several plays) had been on the slates for years, with several competing scripts completed before funding (eventually) materialised. But Tennant was prepared to run alongside the slow train, waiting for the chance to leap board.

“The trouble with doing independent films in Britain is you are never sure you are making them until the very last minute. I wasn’t even sure it would happen until it was actually happening. It all happens on a wing and a prayer and this was not an easy film to get funding for. At first glance, this is not a sexy subject.

“But once Sir Michael Gambon and Elisabeth Moss [Madmen star] came on board you begin to think it’s possible.”

How did he work around this given other commitments such as starring as in ITV drama series Broadchurch, currently back with a new series?

“Yes, that was the problem. I was working on that but I couldn’t just turn up on day one of filming and say [making a quizzical face]:‘What was Ronnie Laing like?’"

“Bob Mullan (writer and director of Mad To Be Normal) was sending material through a lot and the trick was to keep it all on a low boil for as long as possible in the hope you will be on set one day.”

It happened. The results premiered at last month's Glasgow Film Festival, and the film screens at the GFT this week before its general cinema release in early April.

And Tennant plays the two Ronnies perfectly. When his Laing, who died on a St Tropez tennis court in 1989, punches fellow doctors in the face for no apparent reason, you loathe him. When he’s warm and loving to fragile patients, you adore him, realising this was a man holding a mirror up to the reactionaries in the psychiatric world.

“That’s what I would hope would happen,” says the actor, with an appreciative smile.

But after all the research, watching the YouTube footage, reading RD Laing’s books, did he like him?

“It’s hard not to walk in someone’s shoes and not like them,” Tennant says. “Even the most reprehensible people you have a go at, you can’t help but like them.”

Especially if you already realise all our jumpers have little holes, just waiting to tear a little wider.

“Part of being an actor is teasing out the threads of the potential madness that is raging in every one of us,” says the actor. “And you know, but for the grace of God goes any one of us to those very dark corners.”

Mad To be Normal screens at The Glasgow Film Theatre from this Friday until March 27 http://glasgowfilm.org