Prague, July 2007. Sitting round the kitchen table in the late-night gloom of his rented flat, smoking roll-up cigarettes, James McAvoy has a couple of confessions to make.

First off, he admits with a wince, he’s been breaking wind, often and vigorously. He gestures at two hefty tubs sitting on the counter by the cooker. They contain stuff called Ultimate 4 Sustained Protein Build and Mass Gainer Advanced Muscle Building Formula. In the month he’s been in the Czech capital making his new movie, a comic-book adaptation called Wanted, he’s been taking the supplements to aid the bulking up of his musculature, as demanded by this latest role. “And they’ve really helped,” he says – but they do come with the unfortunate side effect of violent flatulence.

“I’ve given up taking them and I’m just trying to eat as much steak and chicken as I can. And I shouldn’t be smoking either because it’s catabolic – it breaks down muscle, as do alcohol and caffeine.”

A half-drunk bottle of Czech lager on the table notwithstanding, McAvoy is taking his training responsibilities seriously, even though he’s finding the regime “near impossible” with a draining shooting schedule born of the fact that he’s in almost every scene in Wanted. “I’m doing four nights in the gym just now,” he says wearily.

Is this muscle-bound hunk really James McAvoy, the actor who (to quote the man himself) has proved so adept at “everyman” roles? Who does a convincing and captivating screen “pleb”? Sure, as Robbie Turner, the noble but doomed soldier-hero of Atonement, the 29-year-old Scotsman delivered a devastating performance, and received a Golden Globe nomination, one of seven garnered by this critical and commercial smash. But in most of the other roles upon which McAvoy has fast built an impressive career – The Last King of Scotland, Starter for 10, Becoming Jane, The Chronicles of Narnia, TV’s Shameless and State of Play – he’s played ordinary Joes who are, at most, roguish. Or, in the case of Mr Tumnus in Narnia, a slightly dodgy faun.

But here he is in Wanted, playing an action hero in an action movie. And here he is, in his first Hollywood film, playing the leading role over established marquee names Angelina Jolie and Morgan Freeman. How did that happen? The answer is simple: McAvoy, having packed a lot of hard work into a short space of time, has become one of the very great actors of his generation. And some of that “greatness” is down to the chameleon-like skills that make him, in this violent big budget thriller, such an intriguing action hero.

Wanted – which, for logistical reasons, is being partly filmed in a Prague studio, under the direction of Kazakhstani horror auteur Timur Bekmanbetov (Night Watch, Day Watch) – is based on a cult graphic novel. McAvoy is Wesley Gibson, a dweeb-ish office worker whose quotidian existence is upended by the appearance of two charismatic strangers, Fox (Jolie) and Sloan (Freeman). They tell Wesley that his father didn’t, as he thinks, die when he was a baby. He perished a couple of nights previously, in the line of duty: he was a top-flight operative with the Fraternity, an ancient group of weavers-cum-assassins who keep the world safe from chaos and take their orders from a Loom Of Fate, a fabric “through which the story of the world is told”. Now Wesley, with the aid of super-fast skills he wasn’t aware he possessed, must avenge his father’s death and save the world.

Something like that, anyway.

In Wanted, all of this makes for kinetic thrills as the Fraternity ride on the roofs of trains, bend the trajectories of the bullets and do unspeakably violent things to unspeakably violent bad guys. So far, so The Matrix-meets-The Six Million Dollar Man. What made McAvoy – one of our most astute young actors – take such a, you might think, sci-fi-silly role? Typically, the actor offers perceptive reasoning.

“I liked the central idea of the character. How do you have someone perform heroic deeds who’s coming from a place of apathy? For me that was the most interesting thing. As opposed to all the multidimensional realities and shit like that. That’s great, all that stuff. But it’s the same as all these films: if they’re any good it’s because the characters are interesting. Not because of their superpowers.”

But still, being a hero has its perks. And this is McAvoy’s other confession. Yes, in the line of duty, he has had to kiss Angelina Jolie.

“It’s the only physical contact we have in the entire film other than when she beats me up – which she does a lot.”

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