Ewen Bremner: Are you having a good war, Ewen?

A common complaint about war movies is that it can be tricky in the heat of battle to distinguish one soldier from another. In Ridley Scott's Black Hawk Down, about the US military's attempt to gatecrash the Somalian civil war in 1993, the camerawork is so manic, and the images so meticulously untidy, that the task of keeping track of who's dead and who's not becomes even harder. Yes, the Pretty One is still alive. They're hardly going to kill off the Pretty One, are they? And the Handsome Psycho is still out there being a damn foolhardy hero. As for the rest of the grunts, with their interchangeable close-cropped scalps and crumpled fatigues – well, all you can say is thank goodness for the young Scottish actor Ewen Bremner. It's reassuring to see a face that stands out in the crowd, a face that would be missed if it suddenly wasn't there.

In moments of panic, of which Black Hawk Down has plenty, Ewen Bremner's brow folds into a deep and spectacular crease, which raises his eyebrows into his hairline, lifts his chin into his mouth, and seems to hollow out his already emaciated face. No, since you asked, he doesn't look like a movie star. He is the first to admit it. Of his role in Pearl Harbor, in which he played a goofball with a speech impediment, he has remarked: "I'm just making Ben Affleck look good." Now he is giggling through a mouthful of croissant. "I don't think Ben Affleck needs me to make him look good," he says. "And I don't think they would have given Ben Affleck that role if he looked like me." But while Bremner might never hold Kate Beckinsale in his arms, and kiss her unsmudgable lips in close-up, it has been noted before, and should be noted again, that he is sweetly handsome in the flesh.

Part of the need for reiteration comes from his regular use of deforming disguises or eccentric body language. As the permanently fried Spud in Trainspotting (1995), his untamed goggle-eyes rolled around in their sockets, and he moved like a greased eel. For Harmony Korine's julien donkey-boy (1999), in which he played a schizophrenic, he had his hair dyed black and tightly permed, and wore a set of gold fronts on his teeth. It was only decided the day before shooting that the specially made black contact lenses might be one freakish detail too many.

If Bremner hasn't physically altered himself for a role, then it's a safe bet that he has endured some species of physical or emotional adversity to make his performance complete. In Black Hawk Down, he gets to do some very un-Ewen Bremner things, like hanging out of choppers and running around with guns. "It was chaos," he recalls. "We didn't have time to rehearse with the helicopters, and when they flew in, it was like being sandblasted – a whole storm of darkness and dog shit. At the end of most days, we were all just speechless."

One of his first parts after moving to London to pursue his acting career was also one of his most gruelling: as Archie, a peripatetic, pent-up wee demon in Mike Leigh's Naked (1993), Bremner spent his brief screen time madly stalking around the screen and hollering, full pelt, in the face of Johnny (David Thewlis), who stumped him with the unforgettable question "What's it like being you?"

So what was it like being Archie? "Horrible," says Bremner softly, seeming to turn over sharp objects in his memory. "I remember that film being really excruciating – the discipline and effort involved. But it was a good education. It woke up things in me that were dormant. I'd always been mild-mannered before; I'd never even lost my temper. But as a result of having been forced into that position very masterfully by Mike Leigh, I've since found out that I do have that capacity in real life to get rowdy and angry about things. It's not like it has taken over. But it's there."

He denies that he actively searches for punishing roles, so it could just be coincidence that he has chosen three consecutive films that have offered possibilities for mortal danger – four if you count his spirited cameo appearance in Snatch (2000), which required him to face the consequences of upsetting Vinnie Jones. Bremner's improvisations on julien donkey-boy included pretending to cradle a dead baby in front of the unwitting and understandably alarmed passengers on a bus, but even this must have seemed like a breeze once he reached the boot-camp designed to whip the Pearl Harbor cast into shape.

"I got really fucked-up doing that," he says. Just talking about it makes him shudder, and he suddenly looks so vulnerable that I have an urge to leave the room. "It was physically challenging for everyone, but I was the most feeble of the group. By the end of it, I was a spectacle, I really was. I was hallucinating and everything. They sent someone to check on me every 15 minutes to make sure I was still breathing." I remember the advice that Michael Caton-Jones gave to Leonardo DiCaprio while shooting This Boy's Life: "Pain is temporary. Film is forever." But perhaps it would be best not to repeat this just now. The pain, or the memory of it, evidently has yet to desert him. He looks tiny when he talks about it: no bigger than a crouton in Ben Affleck's soup.

When I tell Bremner that it was weird to see him in Pearl Harbor, he looks delighted. "Oh yeah?" he beams, as though a minor goal has been achieved. He gave a similar response earlier, when I remarked on his unrecognisable appearance in julien donkey-boy – a sincere and satisfied "great!" like an undercover agent proud to have completed a mission without detection. He claims not to consciously reinvent himself, but there is contentment, too, in his new-found mastery of accents, a talent he had previously considered to be beyond his reach. Following his three American movies, he recently added Spanish to his repertoire with the forthcoming BBC film Surrealissimo, in which he plays Salvador Dali.

But I wonder how happy the experience of making Pearl Harbor could have been for anyone, if the tittle-tattle about its dictatorial director Michael Bay is to be believed. I read Bremner a quote from Kate Beckinsale: "It is frustrating, working on a big film like that, if you actually love acting. It is difficult to do what you would normally do on a movie that is made in that particular way." He considers her words carefully. "With every production you work on, there's an aesthetic that comes from the director," he responds finally. "Michael Bay has got his own sense of humour that he wants to capture on film. They may not be the kind of things that I would find funny, but it's his taste. Sometimes you feel like saying to him, 'If you really wanted this to be funny, you'd do it like this'. But then he'll come back, 'No, no, no, THIS is funny, do it more funny. More funny, y'know?'" He has assumed a no-nonsense American accent and choppy, aggressive hand movements. He's become Michael Bay, full of knotted intensity, before my eyes. "And maybe he's asking you to do something that isn't actually funny at all," he continues, back in his own skin. "I haven't got too many illusions about the craft of acting. I like acting if it's play. As long as it's got energy. I don't know if I love it in the way that Kate is talking about." He pauses, then says quite plainly: "A lot of times I don't enjoy it so much at all."

What might stop you from enjoying it? "When I feel like I can't get it," he says, holding his hands out in front of him, as though scrutinising an invisible and incomprehensible script. "When there's something that I can't get my head around." Once again, he lapses seamlessly into a miniature performance, acting out for me this feeling of helpless bewilderment. "Something is missing... I can't... I'm not understanding this..." He clears his throat. "And there are other factors. On Black Hawk Down, there were days when you'd be sitting in a box on a rubbish dump from morning till night doing nothing."

So how do you stay sane in that environment? "That's the trick. I don't know. You've waited three days to do a scene that you were looking forward to doing, and then it finally comes around and you think 'What was I excited about?' "

If Bremner had been bruised by Hollywood just once, his discomfort would be understandable. But when you sign up for two big-budget war movies in a row, both produced by Jerry Bruckheimer, you can begin to look careless or masochistic or both. And yet his frustration seems laced with real surprise. "When the film you're working on is formulaic, it can be an effort to respect it because in the back of your head you know that the people who initiated the project don't really..." He stops. "All they care about is..." He stops again. "I'd better be careful what I say. Some films are initiated because of a desire to communicate. And others aren't."

'Black Hawk Down' is released today