The talk surrounding Shane Meadows's new movie Once Upon a Time in the Midlands is of star casting - of how the director of two accomplished but commercially unsuccessful features has turned to big names (Robert Carlyle, Rhys Ifans) to get his films noticed. And yet its leading lady is no box office draw, and one of the least starry individuals you're likely to meet. You wouldn't guess, from either her profile or her demeanour, that Shirley Henderson, formerly of Trainspotting, Bridget Jones's Diary and Topsy-Turvy, is one of the brightest, most prolific talents in British film.

The diminutive, 35-year-old Henderson is bracingly down to earth. She seems almost embarrassed to find herself in the leading role in Meadows's movie, about a rascal (Carlyle) who returns to town to reclaim his ex (Henderson) from her nerdy boyfriend. The actress still lives where she grew up, in south Fife, where actorly egos are given short shrift. There's a myth about Henderson, reprinted in several interviews, that her career break came on Opportunity Knocks. "It's just not true," she says.

It is true, however, that she cut her teeth crooning Barbra Streisand and Gladys Knight standards at working men's clubs and at Ayrshire's Butlins. At that time, drama wasn't an option: "There was no local group that you could become part of," she recalls. She discovered acting while on a one-year course at Kircaldy technical college; from there she went on to drama school in London.

Then came theatre work and some telly. Trainspotting was her breakout movie, and she has had frequent film work for several years now. Michael Winterbottom cast her in both Wonderland and 24 Hour Party People. Unlike, say, Ewan McGregor, she has a skill for transformation (which may partly explain her low profile). "The hardest thing is to play close to yourself," she says. "I just did a couple of films in Scotland. I hadn't done anything in a Scottish accent for ages. At first it was bizarre. It was like me speaking." So does she like having an accent to hide behind? "No, it's nothing like that, that's too heavy." Henderson can't be doing with self-analysis. "It's just more fun forgetting yourself," she says. "It's dressing up and playing around and being daft."

It's a classic Scots Presbyterian combination: work very hard then play down the achievement. "Shirley was the one actress I met for the part who questioned me intensely about her character," says Meadows. "I'm only now learning about the female characters in my films. I've learned a lot from Shirley."

Certainly, Henderson had strong ideas about this single mum (also called Shirley) who is torn between lovers old and new. "I recognised her," she says. "I knew girls at school who you look at and think, 'They've got all the guys and they don't have to work at it.' And she's a bit like that. But I never know how happy these people are. They seem to have everything. I remember looking at these girls and wanting to be them."

Because of Meadows's improvisatory approach, Henderson had more scope than usual to create the character along her own lines. She had improvised with Mike Leigh, too, on Topsy-Turvy. But having so much creative input can become addictive. "The more you do it, the more you crave it," she says. "It's hard to go from that playful atmosphere to, 'This is your line, and this is where you move.' You wake up every morning with Shane not quite knowing what you're going to do." His approach is to "create a family. He wants us to have a giggle. He wants working with him to be the best time of your life." Other directors have different ways of "getting the performance out of you, and of making you feel safe. It's important to feel safe. Because you're going out on a limb, you're doing daft things that might make you look ridiculous. You've got to feel that that's OK."

Henderson was so focused on her own character, it seems, that she never twigged it was a western parody until the end of filming.

Didn't the Sergio Leone-style title give the game away? "I'm a bit daydreamy," she explains. But her narrow focus has paid off: hers is a fine piece of film acting, an understated portrait of a woman who, as Henderson observes, "never says anything. She never goes, 'I'm so messed up, I can't decide.' She's just quiet about it. That's what I like. You don't even know half the time what she's thinking."

Henderson is frustrated, however, that some of her own films, even the commercial ones, don't get shown in her local cinema. "Topsy-Turvy didn't get to where I live, which seems crazy. 24 Hour Party People didn't come to me either. Someone somewhere along the line makes a decision that people in these places won't want to see those films. My family get frustrated because they think, 'God, we've got to trek over to Edinburgh again to see [Shirley's film]. They just want to go up the road."

The Hendersons will appreciate her next role, then, in the multiplex-friendly Harry Potter sequel. But might Henderson one day go to Hollywood and bypass Britfilm obscurity? "I don't fancy that, really," she says. "I think there's too many other people they can choose from. Why would they pick me?"

· Once Upon a Time in the Midlands is on general release.