Inside I'm Dancing, a bittersweet comedy-drama about two disabled Dubliners who get on with the business of living because they have nothing to lose, opens the Cork Film Festival this Sunday. Actor James McAvoy talks to Donald Clarke about the challenges of acting from a wheelchair, the controvery over casting - and swapping his Glasgow accent for, first, a Cork, then a Dublin one

James McAvoy, the Scottish star of Damien O'Donnell's paraplegic buddy movie, Inside I'm Dancing, has a bone to pick with his producers. "First of all the character, Rory, was from Cork," he says. "For four weeks I'm doing this Cork accent and I was really loving it. We were doing a really thick accent, but the producers thought it was too alienating. So it was changed to Dublin."

Alienating? Were they seriously suggesting that an international audience might have difficulties making sense of that lilting Lee-side timbre? "Yeah, I think so. I can't really tell why they thought it should be a Dublin accent. But Rory was actually very different when he was from Cork. It was annoying."

With an amusing irony, Inside I'm Dancing, a markedly better film than its mawkish title might suggest, has been selected to open the 49th Cork Film Festival this Sunday evening. Those who choose not to boycott the film in protest at the producers' small-mindedness will be treated to a fine performance by McAvoy. The 25-year-old Glaswegian plays an anarchic Dubliner with muscular dystrophy who, after causing several major disturbances in his care home, embarks on an independent life with a more reserved cerebral palsy patient, played by fellow Scot Steven Robertson.

McAvoy is prepared for the question he will surely get asked most in connection with the movie: why didn't the film-makers cast a disabled actor?

"This is not a socially realistic movie," he says. "It is not a Ken Loach film - much as I love those pictures - in which we were improvising our way through the film. Where are you going to find an experienced actor who has Duchenne muscular dystrophy? You are refining your selection procedure all the time when you say that sort of thing. Why not an Irish actor? I have had disabled people ask why a disabled actor wasn't cast.

"Most people would have been happy if it was an actor with cerebral palsy. We wrote the film round Duchenne muscular dystrophy. We are not going to just change that because we can't find an actor with that precise condition."

The compact, baby-faced McAvoy is not quite a star yet. But you will almost certainly recognise him. "The first time I was recognised I had just come down to London a few months after I had done an episode of The Bill and some guy turned to me and said, 'You were on UK Gold last night.' And I thought, yes, I've arrived."

In the last year he has turned up as Bill Nighy's son in the BBC's I, as Steve in Channel 4's outrageous Shameless and as Paul Bettany's brother in Wimbledon. Next year he will have a prominent role in the first instalment of a major movie adaptation of C.S. Lewis's Narnia books, directed by Shrek's Andrew Adamson.

Brought up in the working-class Glasgow suburb of Drumchapel, McAvoy suffered an early trauma as a toddler when his parents split up. In previous interviews, writers have made a big deal of the fact that he was subsequently brought up by his grandparents. This was supposed to be significant in making him the man he is today.

"Yes, apparently so," he laughs. "But kids are so resilient. If you were brought up on the moon you would probably think that was perfectly normal. My grandparents were brilliant. Actually, I think all my uncles and aunties were all a bit annoyed: 'They practised on us and now they've finally got it right with him'."

After a school visit by the Scottish director David Hayman, McAvoy, a polite young man, felt obliged to apologise for the barracking the guest had received during his speech. He was offered an audition in return and has rarely been out of work since.

After graduating from the Royal Scottish Academy of Music and Drama, McAvoy secured a small role in Gillies MacKinnnon's Regeneration and then a larger one in the epic second World War series Band of Brothers.

"It was amazing, but the whole method aspect of it was bizarre," he says. "I heard stories of the drill sergeant yelling at the actors, 'You will call your gun a rifle! We are doing this for the fallen dead', and the actors just snapping back, 'We are doing this so that HBO can make a lot of money and nothing else'."

Inside I'm Dancing offers him his biggest challenge yet. McAvoy is a notably physical actor and it must have been very difficult to suddenly find himself deprived of the tools of gesture and posture.

"It was terrifying for the first few weeks of rehearsal," he says. "But eventually I surrendered to it and decided that the very fact that I can't move expresses something in itself. But, you know, it really gave me nowhere to hide. I couldn't hide behind little actor's tricks. Which I don't think I do too often, but I have caught myself sometimes."

O'Donnell, as we might expect from the director of East Is East, extracts fine performances from his two leads and from Romola Garai as the inexperienced care worker with whom the unhappy Robertson falls in love. In purely physical terms McAvoy seems to have a sure understanding of how to place his body and how his speech might be affected. It helps that the author of the original story, Christian O'Reilly, had worked as a personal assistant to a patient with cerebral palsy, but McAvoy quite rightly felt that he should talk to muscular dystrophy patients and, in particular, to those who suffer from the particularly virulent strain - Duchenne - that afflicts Rory.

"It is hard to ask anybody about their impending mortality, but there was this one guy I did speak to," he says. "I didn't want to make him feel that we were stealing from him. But he seemed very eager for this film to be made and felt that it was important that it get out there. We talked for hours and what really helped was just being with him for that length of time. It would be disrespectful to regurgitate our conversation here."

McAvoy is prepared to say that he identified a passion for living a potentially short life to the full.

"I just don't live every moment as much as this guy does," he says. "That helped me with Rory a lot. That might be because impending mortality is a very real thing for this guy and for Rory as well. I think there is a link there, but I like to think both of them would be like that anyway. It is not right to define somebody just by their condition."

And as he was doing all this more serious research he was still engaged with perfecting that tricky Cork accent. What a waste! "Yeah I was absolutely gutted. It really is a great accent. I'd love to use it sometime, but they've already made Disco Pigs."

Inside I'm Dancing had its Irish premiere at the Cork Film Festival on Sunday and is released next Friday. See page 15 for free tickets to The Irish Times preview