WHEN James McAvoy was filming Becoming Jane in Ireland last summer - he plays a West Brit aristocrat who seduces Anne Hathaway's Jane Austen - some Irish actors on the set were surprised to discover he was actually Scottish.

"Remember when you made Inside I'm Dancing?" "Yeah?" "A lot of us were so pissed off at Damian O'Donnell for casting a couple of actors from England in the lead roles, it became known as Inside I'm English."

As it happens, the 27-year-old Glaswegian is almost as Irish as he is Scottish. "Half my family are from the borders of Scotland and half are from Donegal, " he says.

"Our mum's dad is from Meath and her mum is from Donegal."

Add to that an actress wife, Anne-Marie Duff, whose Irish immigrant parents met in Shepherd's Bush in the 1960s, and it's no surprise McAvoy had little difficulty being convincing in Inside I'm Dancing as a young Dubliner, crippled by muscular dystrophy, who befriends a vocally impaired youth played by Stephen Robertson.

"My character was originally going to be from Cork and I spent four weeks perfecting the accent, " he says. "I got it beautifully but then two days before we started shooting the producers got worried about American audiences not being able to understand. I told them Cillian Murphy didn't seem to have any trouble being understood, but they insisted on making me into a Dubliner instead."

It's become hard to keep up with McAvoy, who for a long time claimed he was an actor only because he couldn't think of anything else to do. He made his breakthrough opposite Duff as her likeable car thief boyfriend in Shameless, Paul Abbott's groundbreaking foul-mouthed Channel 4 TV series about a dysfunctional Manchester housing estate family brought up by an alcoholic father.

He was an American GI in the award-winning Steven Spielberg war series Band of Brothers and put on a posh English accent for Wimbledon and Stephen Fry's Bright Young Things, before playing the heroic faun Mr Tumnus in The Chronicles of Narnia.

"I've been a fan of James for years, " says Julian Jarrold, who chose McAvoy as a dashing young Regency buck in Becoming Jane, having previously directed him in the TV adaptation of Zadie Smith's novel White Teeth. "The challenge was to enjoy the charisma and roguish charm of the character rather than be turned off by someone who is merely arrogant, and James delivered."

This ability to convey darkness beneath a playful boyishness - a gift McAvoy shares with Cillian Murphy - made him a natural to play the naïve young Scottish doctor, Nicholas Garrigan, who falls under the sway of the brutal 1970s Ugandan dictator Idi Amin in The Last King of Scotland. While Forrest Whitaker's barnstorming portrayal of Amin is tipped to win a best actor award at tomorrow's Golden Globes, McAvoy could well be a best supporting actor contender when the Oscar nominations are announced on Tuesday week.

"Basically I play an asshole, hopefully in a way you can still feel empathy towards him but not necessarily sympathy, " he says.

"I'm not an evil man, but just selfish and vain and egotistical. So that when someone like Amin empowers me, by making me his personal physician, that's really quite dangerous. It's quite a change from Mr Tumnus, who does something because he believes it's right. The most important person in Nicholas's life is Nicholas, whereas with Mr Tumnus, he's selfless. He risks his life to save a wee girl."

Mr Tumnus, of course, was a fantasy character dreamed up by author CS Lewis. "He is fantasy, but he's based I believe on people who harboured Jews in Nazi Germany. They risked and often lost their lives trying to hide them and save them. I feel that's the real-life parallel with Mr Tumnus.

There are people like that, but Nicholas is sorry not because he's sorry but because he got caught."

As in the 1980s Costa Gavras thriller Missing, in which a midWestern conservative dad goes to Pinochet's Chile in search of his radical son who has disappeared, McAvoy's doctor provides a western audience with a way into an alien world of kidnappings, assassinations and unspeakable atrocities in which he, and through him, they themselves, become complicit.

"We tend to see the figure of the westerner in Africa as being heroic, and I'm sure there have been good people from the west in Africa who have been selfless, " says McAvoy. "But if you look at our history and our presence there over the last 200 years, the main characterising feature has not been selflessness, it's been selfishness."

Director Keven McDonald resisted pressure to edit The Last King of Scotland to make McAvoy's character more sympathetic.

"Quite often this happens because producers worry that otherwise the audience won't identify with someone. I don't think that's a truthful way to represent the world."

McAvoy was born in Glasgow's Scotstoun but was brought up with his sister on a Glasgow housing estate, Drumchapel, by his maternal grandparents - and his mother, off and on - after his parents separated when he was seven.

"I had a really good childhood, it was all good, " he says. "My grandparents had been through it all five times before, so they didn't have to make some of the mistakes every parent makes. It was a much more equal relationship for us.

Some philosopher said that you're a child and you grow up and then you return to the child. Maybe it was something like that for them.

You reacquaint yourself with the child inside you. So they were very supportive about my acting, even it they weren't necessarily convinced that I was going to be any good at it, or successful. It's only in the last two years that they've really been satisfied that I'm safe and doing well, even though I'd being doing fine for five years before that."

At school in Drumchapel, McAvoy considered becoming a Catholic missionary, but then got talking with theatre director David Hayman when he visited the school and ended up auditioning and getting a part in Hayman's film The Near Room.

"I was terrible in it, " he says. "I only had a small part as the son of a pimp who prostitutes young children, and I was in love with one of his prostitutes. I mean one day to repay David by being good in something for him."

Sam Mendes, who gave him a part in Privates On Parade at the Donmar in 2001, remembered McAvoy - as did Tom Hanks, who produced Band of Brothers - when they were casting Starter For Ten, a comedy about a nerdish 1980s college fresher who whose fantasy is to appear on Bamber Gascoigne's University Challenge quiz show.

"It's nice to have guys like that have confidence in you, " he says.

"It really empowered me and gave me the confidence to go on."

Starter For Ten premiered at last autumn's Toronto Film Festival, along with The Last King of Scotland and Penelope, a modern fable in which he co-stars with Christina Ricci, putting him in the unique position of featuring in three different films at his first festival. "It was the most intense two weeks in my life, " he says.

McAvoy had never read a Jane Austen novel - "we just did Shakespeare at school" - until he was auditioning for Pride and Prejudice. He didn't get a part but director Joe Wright called him back to star opposite Keira Knightley as a working-class boy who becomes victim of a younger girl's imagination in an adaptation of Ian McEwan's novel Atonement.

Apart from playing the lead in Joe Macbeth, a TV update of the Shakespeare play, The Last King of Scotland is the first starring role in which McAvoy has been able to use his natural accent. His ability to do accents has widened the range of roles open to him.

"It means I'm not locked into playing just one kind of person, " he says. "If you don't do accents, it's not the end of the world but doing accents is another tool that helps you to be someone else. If you can only do Glaswegian accents, it just closes doors to the range of roles you can do."

'The Last King Of Scotland' opened on Friday. 'Becoming Jane' opens on 9 March.