The Glaswegian actor bulked up to play 24 characters in his latest film <i>Split</i>. Now the real Mr James McAvoy steps forward.

Meet Mr James McAvoy: actor, dad, divorcee (pending), Scotsman, adoptive north Londoner, Celtic fan, gym lad (if forced), biker, X-Man, whisky enthusiast (lapsed, by necessity), swearer (he fucking loves swearing). Actually meet Messrs James McAvoy, plural. Very much plural.

In his new film, Split, Mr McAvoy plays a man with dissociative identity disorder. That is, multiple personalities reside within a damaged individual known as Kevin. He is peopled by more than 20 personalities. Given that Split is the creation of Mr M Night Shyamalan, the writer/director/auteur behind deathless chiller The Sixth Sense – and several other almost as scary but not quite so good films (Unbreakable, Signs, Lady In The Water) – these personalities are not all cuddly and kooky. A couple of them are downright malevolent, notably Dennis.

Dennis is the first Mr McAvoy we see on screen. He’s a bulked-up, buttoned-up, neat-freak nutjob, who gets the party started by gassing and kidnapping three teenage girls, then imprisoning them in an isolated hellhole somewhere in Philadelphia’s urban sprawl. The only other residents are the alternate Mr McAvoys, who include a camp New York fashionista called Barry, a prim Englishwoman named Patricia and a lisping nine-year-old boy who goes by the name Hedwig. All of them played with screen-burning relish by a 37-year-old bloke from Glasgow.

For Mr Shyamalan, Split is a long overdue return to form. For Mr McAvoy, it is a tour de force, a chance for him to flex in one project the varied acting muscles he has developed in a brilliant, and brilliantly diverse, career. That near-two-decade run ranges from his breakthrough in Channel 4’s Shameless to his Big Hollywood Moment in the X-Men prequel blockbusters, from Mr Ian McEwan adaptation Atonement to Mr Kevin Macdonald’s The Last King Of Scotland.

Pulling together all those different acting styles, Mr McAvoy is the heart and soul of Split. And its anchor. Without his steady hand, the never knowingly understated Mr Shyamalan’s high-concept horror could have ended up, well, more silly than chilly.

We meet on a grey winter’s day in central London. Dressed in a  T-shirt and jeans, Mr McAvoy rocks up to Fitzrovia’s sleek-chic-discreet Chinese restaurant Hakkasan. The high-end Michelin-starred establishment was my suggestion. His was to meet in a chain coffee shop on chaotic Tottenham Court Road, within earshot of the giant Crossrail works. But Hakkasan means Mr McAvoy can eat properly – something he’s not done yet today. He’s spent the morning a few hundred yards away, at Universal’s offices, geeing up the staff troops in a Q&A with Mr Shyamalan.

Mr McAvoy became acquainted with eating properly – for properly read excessively – while filming Split. To play at least some of the characters, he had to get ultra fit.

I’ve seen him do this before. I met him once in Prague, in 2007. He was living a lonely existence in a dreary flat while he filmed Wanted. In the bonkers comic-book adaptation, Mr McAvoy played an office schlub turned super-assassin. His mentor: Ms Angelina Jolie. His tormentor: the buckets of dietary supplements he was mainlining, the better to build up his musculature. You needed biceps like Conan even to lift the tubs of Ultimate 4 Sustained Protein Build and Mass Gainer Advanced Muscle Building Formula. They looked vile.

When I remind him of this, Mr McAvoy winces, because I also remind him of the supplements’ side effects. They made him fart like a flatulent Friesian.

“On that job, I didn’t go out a lot,” he recalls. “I was on set 12, 13 hours a day, and I was in every scene, so it was different. With Split, I had less time to get it done, and I did it in a different way. I googled ‘good way to put on muscle quick’, just went into a gym and did it myself: a power-lifting routine called 5x5. You do five different exercises four or five times a week, and they’re huge, big exercises. Pick something up and do that,” he says, extending his arms above his head, “then put something on your back, sit down and stand up.

“It’s super-simple, but it works every muscle. And I’d eat 5,000, 6,000 calories a day. Instead of eating two eggs in the morning, I’d eat eight. Then a snack of chicken breast. Then two chicken breasts for lunch, and then a steak for another snack. Then two salmon steaks for dinner.

“So, in a relatively short space of time I got relatively bigger,” he laughs self-mockingly, acknowledging (not for the only time today) his relatively compact stature. Mr McAvoy is 5ft 7in, but, even when he’s not mainlining steaks, has a width that reveals an enthusiasm for boxing. “I wasn’t huge,” he clarifies, “but I was quite chuffed with myself.”

It was, again, a solitary experience. The loneliness of the long-distance calorie chugger. One suspects that Mr McAvoy relishes this utter submersion in, surrender to, the process of building a character by rebuilding himself.

Last year, filming in Madrid, he imposed on himself similar body sculpting, albeit in the opposite direction. To play a hostage in Mr Wim Wenders’ upcoming romantic thriller Submergence (his leading lady: Ms Alicia Vikander), Mr McAvoy dieted hard in pursuit of an appropriately malnourished physique. Then, to shake off the trauma of five days’ filming inside a metal box, he went on a 13-hour bender in the Spanish capital.

To clarify, it wasn’t a beery night. Ever diligent, even when on the lash, Mr McAvoy didn’t want to ruin his diet plan by loading up on fizzy alco-sugar. So he stuck to tequila and vodka. Lots of it, he recalls cheerfully. Previously, he adds, that might have been a whisky night.

“That used to be my drink – a peaty Talisker, or a Laphroaig,” he says. “But I find that I can’t drink too much whisky any more. More than one or two now and I get a bit leery, a wee bit fighty, a bit chippy, looking for an argument. And I didn’t like that. So I mostly stopped drinking it. My problem is, if I have it in the house, I’ll tan the lot.

“I’m a consumer,” he shrugs, confident enough in himself to be able to talk so freely. “If it’s in front of me, I’ll fucking do it. I’ll consume it. I’ll take it, whatever it is. I’ll have a go… And I don’t know what that is,” he frowns. “I still drink, and sometimes have a lot of drink,” he says, brightening, still an out and proud Scotsman (I’m allowed to say that; I’m one, too). “But I just don’t want to have alcohol in the house any more.”

The elephant in the room, meanwhile, is nothing to do with Mr McAvoy’s still very buff trunk. Last year it was announced that he and Ms Anne-Marie Duff, the Shameless co-star he married in 2006 and with whom he has a six-year-old son, Brendan, had separated. Asking, politely, diplomatically, how his life has changed over the past 12 months, solicits an elegant response.

“Ha, ha!” Mr McAvoy begins, laughing good-naturedly, knowing full well what is really being asked. “My life has changed massively.”

Post-split, he has not relocated to LA. In almost 20 years’ working, he thinks he’s spent in total “10 or 11 weeks in Hollywood”. The fact that he knows the number shows his evident, and justifiable, satisfaction at having managed to build a career without having to follow the money.

He has no desire to up sticks anywhere in pursuit of work. The work can come to him, thanks very much, which it invariably does, whether on the London stage (his star turns include Macbeth and The Ruling Class) or via intense bursts of filming. First and foremost, he wants to be near his son. So, yes, while he has moved into his own place, it’s not far from the erstwhile family home in Crouch End.

“At the same time,” he continues amenably, “so much has stayed the same. One of the things that’s stayed the same is that I still don’t talk about my personal life, really. Me and Anne-Marie, when we were together, it was our policy not to speak about each other in public. We rarely broke that and if we did, it was for tiny things – ‘Yes, we are cooking turkey this Christmas’ – and that policy still stands. Even separated, we’re still respectful of each other and committed to doing that publicly and personally.

“But yeah, things are really good,” he grins again. But, of course, he knows the score. “Which is a rubbish, pat answer.”

In a way, as an actor – or as an interviewee – Mr McAvoy is a product of his upbringing. After his father walked out on his mother when he was seven, he was largely raised by his grandparents in a loving, working-class Glaswegian household. He learnt to push himself, and speak for himself, and make his own chances. When Mr David Hayman gave a talk at young Master McAvoy’s Catholic comprehensive, he buttonholed the actor/director afterwards. Any chance of some work experience?

From school, he secured a place at the Royal Scottish Academy of Music and Drama. Some 20 years on, Mr McAvoy initiated a scholarship at the college, which is now named the Royal Conservatoire of Scotland. On his behalf, they administer and disburse money “as they see fit”, based on age, talent and need.

“It’s about art in education,” he explains of a programme that’s now in its third year. “I wanted to be able to fund kids who couldn’t afford to do a week’s summer camp or whatever it was, so they have some exposure to what I think can be life-changing moments. I don’t care if any of them become actors; it’s not about that. It’s just about putting yourself out there and challenging yourself and expanding your horizons.

“I felt that would be useful, and something I’ve been going on about all the fucking time when people ask me about the class divide in acting. I’ve got strong opinions about that, and this is a way I can actually physically act upon it, in a small way.”

The initiative began after the picky, choosy, principled Mr McAvoy was offered a Prada advertising campaign. He was about to shoot back his usual “no thanks”, but he credits Ms Duff with making him get real.

“The money was incredible,” he says. “And Anne-Marie kept saying to me, ‘Are you gonna turn that down?’ ‘Yeah I’m gonna turn that down. I’m not a salesman.’ ‘You’re being a dick. Do something with it. Give it to charity.’ And I was like, ‘Ding!’ About four minutes after she said that to me I was on the phone to the agent. ‘Yeah, I’ll do it.’”

He gave one third of the “incredible” fee to Unicef, one third to Retrak, a Ugandan organisation that helps street kids (he encountered the charity while filming The Last King Of Scotland in Africa), and one third to the new scholarship.

Would he take another ad campaign just to top up the scholarship funds?

“Maybe. Depending on what it is. I don’t think I would do a Nespresso,” he smiles of the itsy-bitsy coffee cartridges for which Mr George Clooney is a, well, salesman.

Professionally speaking, Mr McAvoy has barely paused for breath since graduating from drama school in 2000. As well as Split and Submergence, this year we’ll also see him in The Coldest City, a Berlin-set spy thriller in which he appears alongside Ms Charlize Theron. And then… a fourth X-Men prequel spectacular? Possibly not. Neither he nor either of his co-stars, Mr Michael Fassbender and Ms Jennifer Lawrence, is contracted for a new film.

“They certainly haven’t asked me to do another yet,” he says. “I know they’re writing another [original] X-Men movie. Whether they’re gonna make it or not, I don’t know. And I know they’re looking at doing some spin-offs as well, that I may or may not be involved in.

“It’s all up in the air at the moment,” he concludes, blithely unfazed by the not knowing. “I may end up being in fucking 20 X-Men movies in the next five years. And I may end up being in none.”

How does Mr James McAvoy feel about that? He’s cool either way.

Split is out 20 January