James McAvoy is not one to shy away from tough gigs, but a string of testing roles – the latest in M Night Shyamalan’s Split – have left him at a career crossroads. Gemma Dunn finds out more

There's no challenge too great for James McAvoy. The Scottish actor won over American audiences with his critically acclaimed breakthrough performances in The Last King Of Scotland and Atonement; he drew rave reviews for his gruelling 88-show portrayal of Macbeth in the West End; put in a gasp-worthy performance as a drug-fuelled cop in Filth - a film so shocking it almost didn't get released - and gained heart-throb status as Charles Xavier in the X-Men series.

Attacking every role with gusto is the McAvoy way, but his latest showcase - M. Night Shyamalan's big-screen comeback, Split - expends everything he's got.

Returning to the captivating grip of Shyamalan's The Sixth Sense, psychological thriller Split delves into the mysterious recesses of one man's fractured, gifted mind. Suffering from Dissociative Identity Disorder (DID), Kevin, played by McAvoy, has evidenced 23 personalities - expect to meet Barry, Dennis, Miss Patricia, nine-year-old Hedwig, and others - to his trusted psychiatrist Dr Fletcher (Betty Buckley). But there remains one still submerged, who, once materialised, is set to dominate all the others.

So, that's basically 24 characters McAvoy's tackling then. "I knew it was a challenge, but I generally think I can do anything!" the 37-year-old declares, laughing, before quickly adding, "Well, not anything, but when I'm reading a script, if I'm getting loads of ideas, I feel confident. There are two scenes in it I didn't feel confident about - one of them because it made me feel bad when one of the guys was doing something bad to one of the women in it, and then another scene where I have to portray eight of the personalities that live within one body all coming out in a moment of high anxiety.

"That was pretty devilishly difficult," he admits. "But unless it says: 'He walks into the room, he's 6ft 5in and he's built like a brick sh*thouse, you know, then I'm fine. I feel like I can do whatever."

McAvoy - sat cross-legged in black jeans, a maroon sweatshirt and casual jacket - is on good form. While there's little small talk from the notoriously private star, he's instantly likeable. Business-like maybe, but charming too.

As anticipated, personal questions are strictly off-limits (only last year, McAvoy announced his separation from wife and former Shameless co-star Anne-Marie Duff, with whom he has a son), but ask the Glaswegian - his groomed beard doing little to mask his youthful looks - about his art, and he lights up. He lists Split's many twists and turns, Shyamalan's bold commitment to creating and self-funding the project, plus the opportunity for an actor "to radically change what you're thinking, who you are and what makes you in a moment", as the appeal for signing up.

And as for his tour de force of physical transformations between one character and the next - bulging veins, rippling muscles and an impressive female posture for Miss Patricia, no less - he says he simply went with his gut.

"You just know when you feel like you're telling the right story," McAvoy explains, passionately. "One of the things I think about actors, and certainly myself as an actor, is that I am aware of what I'm emitting. I am aware of what I'm doing with my body and with my face. I don't need a mirror on set to show me if I'm conveying the right story with my every move; you just know, you feel it. You feel it in your throat, you feel it in your face, you feel it in your shoulders, and you feel it in your bum...

"You do!" he quips, noting my surprise. "I'm not even joking, you feel it everywhere when you're keying into the right thing for the right character."

With a career that has barely come up for a breather since he graduated in 2000 from the Royal Scottish Academy of Music and Drama (now The Royal Conservatoire of Scotland), there's no doubt he knows his stuff.

Nearly four years ago, ahead of the release of Filth, McAvoy told The Guardian: "I'd like to be a character lead; that's what I'd like to be. The kind of part I played in Atonement is a leading man, and Filth is a character lead. Actually, what I like to do is be the guy who's in the most scenes, has the most lines, and has the best part! But hopefully not have to be too straight."

He's got his wish 24 times over with Split.

So does he agree with the many reviewers hailing this 'the role of his career?'

"Pfft, I have heard that a few times in my career," he retorts. "It's nice of people to say that - I think they mean it as a compliment and I accept it as a compliment... For me personally, I hope it is the role of my career that I have had so far, and I hope that the next one will be the role of my career again. Do you know what I mean?" he asks, smiling. "Because then it only gets better and better and better.

"When I did Filth, and Macbeth on stage, those two were the defining moments of my career for me. Not necessarily for anybody else, but for me. And what it left me, unfortunately, was not really wanting to do anything else, because nothing else that came in for me touched it, in terms of complexity or challenge. Nothing was as dynamic as those two productions or characters were. At the moment, I've had a very diverse year, and I'm in that situation where I'm going, 'I don't know what to do, nothing is really interesting me'," McAvoy confesses.

"I can't take a step back, but maybe in a year's time, if I haven't worked because I keep waiting for something else that's a step forward, I'll have to take a step back. But I hope not."

Split is in cinemas now