THERE are times when it’s difficult to figure James McAvoy for a Hollywood star. Like today, as he arrives at a London hotel to beat the drum for his new film Victor Frankenstein. He doesn’t dress designer – just jeans and a maroon jumper. He doesn’t wear flash jewellery – just a black digital watch. And he doesn’t even look ripped – just slim and, at 5ft 7, slight. But a star he is, from heading up the revamped X-Men franchise to sharing the screen with Angelina Jolie (in Wanted) and Keira Knightley (Atonement).

The last time I met McAvoy, he was knee-deep in Filth. To be precise, he was sitting in a chic Knightsbridge flat doubling as a makeshift editing suite. “That was Trudie Styler’s place,” he grins. Along with McAvoy, Sting’s better half was one of the producers on Jon S Baird’s 2013 take on Irvine Welsh’s novel about a wee man on the verge of a nervous breakdown. McAvoy, as Filth’s sick, sadistic copper Bruce Robertson, got some of the best reviews of an already-impressive career.

It may not have presented a tourist board-approved image of Scotland but it took £4 million in the UK. “We earned our money back instantly,” says McAvoy, “which never happens.” Never mind that it didn’t go down too well in America – “I think one guy and his whippet went to see it”. For a first producing experience, it was an unreserved success. What’s more, it’s helped shape McAvoy’s recent output, which has also seen him racking up respect on stage. “It’s been a strange few years,” he muses. “There have been a lot of mental people.”

It all began with Danny Boyle’s 2013 thriller Trance – not a major hit but one that “people in the industry saw”. His performance as Simon, the memory-addled auction-house employee caught up in a violent heist, showed that he could play “mental” just as well as he could soft-hearted – something that defined the beginning of his career in films like rom-com Starter For Ten and the blockbuster CS Lewis adaptation The Lion, The Witch And The Wardrobe, as the gentle faun, Mr Tumnus.

Since then, McAvoy has been wallowing in the mire; notably as a boozy, depressed Professor X – his second turn as the younger wheelchair-using mutant, first played by Patrick Stewart – in X-Men: Days Of Future Past. On stage, he’s been Shakespeare’s blood-soaked Scottish would-be king Macbeth, as well as a paranoid schizophrenic who believes he’s the Messiah in The Ruling Class. The latter role just won him Best Actor at the Evening Standard Theatre Awards last week, a performance so electric, as one reviewer put it: “He makes you feel he can do anything.”

Now 36, he’s back on screen in the title role in Victor Frankenstein, a weird and wild take on the Mary Shelley classic novel about a scientist dedicated to the godly act of creation. Bursting with energy, the script not only draws from Shelley’s characters, it also explores their place in pop culture. McAvoy can be heard yelling “It’s alive!”, rather like Colin Clive’s doctor in Universal’s classic 1931 effort Frankenstein, while Daniel Radcliffe plays Victor’s assistant Igor, inspired by the hunchbacked assistant in the same film.

McAvoy was hooked on the script the moment he read it. “I just kept turning the page,” he recalls. “I had a silly grin on my face, which was one of the things I didn’t expect when reading a script called Victor Frankenstein.” He points out that screenwriter Max Landis was inspired by Facebook drama The Social Network, “becoming interested in young men at the forefront of technology changing the way we live”. So does that make Frankenstein a prototype for the social media site’s Mark Zuckerberg? “Yeah, pretty much – except he was doing it with flesh instead of the internet.”

McAvoy is not the only one flying the flag for Scottish talent in the film. It’s directed by the Bellshill-born Paul McGuigan, recently Bafta and Emmy nominated for his work on the BBC’s Sherlock. Curiously, McAvoy met him years ago, helping record some voiceover work for McGuigan’s 1998 adaptation of Irvine Welsh’s The Acid House. “I’m one of the many voices – maybe in The Granton Star Cause [one of the three Welsh short stories adapted], you can hear me screaming me in the background!”

While Victor’s streak of insanity recalls McAvoy’s recent body of work, it’s also yet another upper-class character for his CV. “I’ve made an entire career out of playing posh people,” he says. Indeed, for an actor who grew up in the working-class area of Glasgow’s Drumchapel, he’s never been typecast – turning him into an unwitting spokesperson on the subject. Earlier this year, McAvoy weighed into the discussion regarding opportunities for British actors, after the privately-educated Benedict Cumberbatch and Eddie Redmayne were a force at the Oscars.

"Nobody has got anything against an actor who is posh and is doing really well," he said in February. "But we are real worried about a society that doesn't give opportunities to everybody from every walk of life to be able to get into the arts, and that is happening. That doesn't affect us right now, but it will affect us five years from now, 10 years from now, certainly further down the line."

When I bring the subject up now, McAvoy wants to clarify one thing. “I don’t care where the actors come from. I don’t care what [their] social background [is].”

As he stated back in February, his concern was living in a “society that doesn’t give opportunities to everybody from every walk of life to be able to get into the arts”. He reiterates in front of me. “I do really care about children who will never become actors or performers not being given the same access to artistic education. That’s the thing that can elevate you. That’s the thing that can make you transcend your boundaries. I love sport and I love that you have to do maths as well. But exposing people to art is an elevating experience. You take that away from people and you shut their horizons down. And that’s really sad.”

He seems genuinely surprised and pleased when I explain that his comments stirred up much positive debate, particularly in Scotland, on the subject of opportunities for working-class youngsters. (In an essay on class and the arts in this newspaper, Elaine C Smith wrote that McAvoy had articulated the problem "brilliantly".)

He leans forward, takes a breath and delivers a long, passionate reply about arts funding under the current Conservative government, and the demands for British film to turn a profit. “Quite often, you get people like David Cameron and other politicians saying: ‘How much is it worth?’” he says. “It’s not about making money. It’s about putting money into arts and education particularly to make the whole country a healthier place.”

Some may dismiss this as idealistic, but McAvoy sees the arts as the way to “liberate people” from the more mundane aspects of life – work, bosses, taxes. “I’m not saying I want anarchy in the streets and that art is going to be the way to do that. But I do think that unless you’re going to come out and say everybody is stuck in this system, a first-world caste system … unless you’re going to be honest about that, stop lying. I don’t think I can take the lie that we have unlimited opportunity in this country because we don’t.”

Certainly, McAvoy is a fine example of succeeding in spite of his surroundings. His father James, a roofer, abandoned the family when he was a young lad. McAvoy, his sister Joy and their mother Liz, a psychiatric nurse, moved in with his maternal grandparents. Raised Catholic, it was a firm but fair household. Not that he ever had a desire to misbehave, he says. Or the opportunity. As he once admitted: “I was never let out of the door on my own until I was 16.”

In his youth, McAvoy dreamt of being a doctor, a pilot, joining the navy and even, “for five minutes”, considered becoming a missionary. “You just try to think of something useful to do,” he shrugs. In the past, McAvoy has stated that “if you grow up in a dodgy area”, reality can often beat down on lofty ambitions. But not him. “I was really lucky,” he nods. He pays tribute to two music teachers at his school, St Thomas Aquinas Secondary in Jordanhill, who ran “an incredibly liberated department that made you feel like you were allowed to be expressive and not just be a ned”.

His second stroke of luck was meeting actor-director David Hayman, who was visiting his school to talk about acting. The other children jeered but McAvoy was entranced. Emboldened, he asked Hayman if he could have some work experience; six months later, he was offered a small part in a film Hayman was directing, 1995’s The Near Room. Again, he credits his teachers. “If it wasn’t for those people giving me a certain level of openness and confidence beyond the closed-down, neddy way I was living my life before that, I wouldn’t have got that film.”

McAvoy went on to study at the Royal Scottish Academy of Music and Drama, and after graduating, began winning TV roles – notably in the BBC drama State Of Play and as the cheeky southern car thief in Shameless, where he met future wife, actress Anne-Marie Duff. “I’ve always been intimidated by how good an actress she is,” he once told me, and you suspect the feeling is mutual. The two married in 2006, and she gave birth to their son, Brendan, four years later.

The family live in North London, and McAvoy recently told an interviewer that he only comes back to Scotland a few times a year. He did vote in last year's independence referendum, however, and though he didn't say where he marked his cross, he admitted he is "very much for" independence "at some point", and thinks it will happen. He added that he expects a Gwyneth Paltrow-style "conscious uncoupling, rather than a messy break-up", and that "we need to still be in some kind of partnership with the rest of the island".

In interviews, McAvoy and Duff rarely talk about each other or their son (no exact birth date was ever made official) to the press. But when our conversation moves on to the subject of X-Men – McAvoy returns next year in X-Men: Apocalypse – he can’t help himself. Playing the character earns some serious brownie points. “When I’m looking through sticker albums or comic books with my kid, he’s like: ‘What’s X-Men?’ And I say, ‘I’m the boss of them!’ That’s kinda cool!” To a five-year-old, it undoubtedly is. In the meantime, McAvoy is about to shoot The Sixth Sense director M Night Shyamalan’s “proper secretive” new film Split before Cold War drama The Coldest City. “The work I have coming up is quite dark.” It really is a case of no more Mr Nice Guy.

Victor Frankenstein is on general release