Girl on top Trainspotting made her, a deodorant ad sustained her and Gosford Park redeemed her. Kelly Macdonald could scale the dizzy heights if only she had the inclination. So what's stopping her?

''It's the schoolgirl thing,'' Macdonald says with a what-can-you-do shrug about her role in male fantasies while drinking a coffee in central London, neither the spangly dress nor the angular hairstyle much in evidence. In fact the only constant from her turn as Trainspotting's Diane - the role that marked her breakthrough and, until Gosford Park at least, the high point of her acting career - is that Minnie Minx of a voice, still present and Caledonian despite five years' residence in London.

It is a Friday morning in April and she has arrived in a short skirt, patterned tights, plain black top, simple jewellery and pleasant demeanour. The hair, now highlighted, is longer, shoulder-length, but held back and up off her face by braids. There is a mark on her nose, as if she has been wearing a monocle (though this is unlikely), but otherwise she is fresh-faced and make-up free. It's a sharp, expressive face. At 27, the girl she was is still evident, but she looks grown-up now. She says she feels grown-up too. ''I am getting there. I am. I like being the age I am. You get onto an even keel. I can't believe that I was slightly freaking out that I was turning 25.''

It is appropriate then that the reason for our latte-fuelled conversation is an adult role, one of her first, in State of Play, a new political thriller on BBC1 written by Auntie's favourite scriptwriter Paul Abbott (the man responsible for Clocking Off and Linda Green). In it, she plays Della, an idealistic journalist who finds herself embroiled in a murder investigation alongside fellow hack John Simm and politician David Morrissey. She wanted to do it because Della is ''kind of a grown-up'', she says. ''She's got a proper job. You know what her job is. She's career-minded.''

Given that she has spent most of the last eight years playing a procession of naive girls, Della obviously feels like a step forward professionally and maybe personally for Macdonald. ''I felt slightly that I was dressed as a grown-up still. But I am. I am the age of the character, so it's fine.'' She almost sounds convinced.

At the PR's suggestion, we meet in London's Charlotte Street Hotel. I arrive early to find it something of a meeja haunt, all suits and laptops. As I sit with a pot of Earl Grey tea for company (a mere (pounds) 3.84, though you do get a couple of shortbread biscuits with that), all around me, money is being talked, deals are being done. There seems to be a film crew somewhere else in the building, hence the young woman with the clipboard and the headphones hovering at the hotel entrance. To my left a producer and his director are talking percentages and schedules. ''We start shooting in January,'' the producer says. On the other side, a table of music execs are discussing a video that's just a bit ''too kitsch'' for comfort. ''It was just never going to get shown,'' one says. ''All those girls with tassles on their tits ... ''

Frankly, I can't think of a less likely place to meet Macdonald than a room full of players and hustlers. While her Trainspotting co-stars Ewan McGregor, Robert Carlyle and even Jonny Lee Miller used Danny Boyle's 1995 film to kick-start high-powered careers, Macdonald has tootled along, always in work but, until Gosford Park anyway, rarely in mind. The few times she has had lead roles - as a call girl in Stella Does Tricks, or as a worker in a bingo hall in House! - the films have been conspicuous by their subterranean profile. As for the rest of her CV, it has often been a case of the bigger the film - Elizabeth, Cousin Bette - the smaller the role. No wonder then that her place in the public imagination is still so readily associated with Trainspotting (or maybe for that deodorant ad she fronted a few years back).

Macdonald, though, isn't bothered in the slightest. Sitting in a cafe around the corner from all those media types where the only background noise is the sound of the espresso machine, she tells me she is perfectly happy with her career trajectory. Trainspotting is not, it seems, a weight around her neck.

''God no. Something that hangs around your neck is if you've murdered a person. Trainspotting put me up there and then you do other jobs that you get a lot out of, but maybe didn't do the business at the box office, and then you get something like Gosford Park and that brings you up again and I think that's a perfectly reasonable course for things to go.'' I wonder if she's pushy enough. ''You can talk to my agent about that. I'm slightly lazy, which probably means I don't push myself but ... hmmm.'' She pauses, starts again. ''I don't think it's about pushing myself. I'm known enough over here. America is a whole different thing that I'm not quite sure of. But I'm known by casting directors so I know that if a part comes up that is within my age group, then they'll put my name down on the list. And you kind of have to trust in that. I get really impressed when I read about actors who have harangued directors into giving them a part. I think that's quite brave and brilliant.''

But not something Macdonald would do, it seems.

She loves acting, she says. ''It's exciting and the bonding you get when you're making a film with the crew and the cast is great. You're all sort of working on this one thing, all trying to achieve this great thing, and it's quite nice. It's like being in the cool gang at school.''

Not that Macdonald would know what that felt like exactly. She was always just on the periphery of the cool gang at school in Glasgow. She grew up on the south side of the city with her younger brother David and mother Patsy. It was an ''average childhood'', she says, but won't elaborate on what that means. Her father was a painter and decorator. Macdonald was seven when her parents split up. Reading cuttings, I'd got the impression that he had walked out on them. She gets animated when I bring it up. ''He didn't walk out. That was totally ... that was a journalist who made that up. He didn't walk out.''

So they just separated, I say. ''Yeah,'' she says. Are you still in touch with him? ''No.'' Is that a source of regret? ''It's not even an issue. It's just the way it is.'' That is as forthcoming as she gets about her family. She doesn't evade questions, just answers them with a clipped yes or no. A better example of passive-aggressive I can't think of.

At school she says she was an okay student. ''I did well at the subjects I was interested in, which was often written on my report cards. 'Kelly doesn't try if she isn't interested.' And I'm still guilty of that.''

She left home at 17 and started working in bars and cafes. Acting was, at best, a vague impulse. ''I just wanted money to be able to go out at the weekend. I was kind of putting off thinking about what I wanted to do because at the back of my mind I knew I wanted to get involved in drama somehow, but I knew that would mean putting myself in a position where it might be a bit scary - having to audition for drama school or whatever. So basically I was just faffing.''

It was a friend who pushed her into turning up for the Trainspotting audition. She remembers getting the call from the film's director Danny Boyle to tell her she had the part. ''He kept me hanging on the phone not knowing. It's like that thing they do on Pop Idol. 'You've done so well to get this far.' Then he said he'd like to offer me the part of Diane. My mum wasn't home yet and I remember seeing her car driving up and running outside to tell her and jumping up and down in the street. It was very exciting.''

Terrifying, too, she admits, when filming finally started. ''But that's the best thing about things like that. They're only so good because they're so scary. It's like bungee jumping or something. The adrenalin is just incredible because it's terrifying and it still sort of is.''

Her subsequent career could be described as something of a slow-burn, but all Macdonald cares about is the fact that it is still burning. Indeed, this year will see her rack up significant screen time. After State of Play she has two films coming out in the autumn. She plays Peter Pan, or rather an actress playing Peter Pan, in Neverland in which she appears alongside the likes of Dustin Hoffman, Johnny Depp, Julie Christie and Kate Winslet. A Miramax thing, she says. The sort of thing that might boost her profile in LA, which, she admits, is even lower than it is in the UK. ''I've got agents out there and they've probably forgotten that they represent me.''

She'll also be seen in Intermission with Hollywood's favourite new flavour Colin Farrell and fellow Scot Shirley Henderson. ''It's an ensemble cast,'' Macdonald says of the Irish film. ''They're my favourites. There's no pressure on you to carry anything.''

It's a throwaway comment, but so revealing. Macdonald, you feel, likes to hide in the background. Not that she has never aimed high. She auditioned for Gwyneth Paltrow's part in Shakespeare in Love and Nicole Kidman's in Moulin Rouge, but she shows no bitterness about being passed over. ''There's reasons for not getting them and you have to go with that. But there's nothing, though, that I haven't got that I thought I should have got.''

Anyway, she says, it's amazing the things she's been in that people have actually seen. ''You think they were just in the cinema for two days and then gone, but people do actually see things and are aware of the work that you do and I've never got a job because of how well a film's done. That would be ridiculous. I wouldn't work at all.

''There's nothing I would change about the way things are going,'' she adds. ''It's not like I'm chomping at the bit for the big meaty dramatic parts. I'm much happier to see what happens.'' Given the choice she would rather play comedy anyway. ''Yeah, I like lighthearted, sweet things. I'm such a sucker for romantic comedies.''

How ambitious are you? ''I'm ambitious. I don't aggressively pursue anything to do with my career, but I want to do good stuff. I suppose that makes me ambitious. I want to keep sort of getting better, so yeah. But it's not that I do a job because I want it to be a huge success. That's not my job. That's for the producers to worry about.''

Okay, perhaps, she is ambitious. But not driven. She finished filming State of Play in the middle of January and she's done nothing since, which is perfectly fine, she says. She's got her tax money set aside and she's just spent her time doing up the kitchen of her North London home. ''It's a love affair,'' she says of the process.

This is the flat she shares with her fiance Dougie Payne, the bassist from Travis. She is as forthcoming about her beau as she is about her family. They grew up a few streets apart, but never met until both were in London. The first time they went out together was to the pub. ''Just a normal first date,'' she says. Famously, he proposed to her at the top of the Empire State Building. When is he going to make an honest woman of her, I ask. She says she doesn't know. There are no plans, although the rumour is a wedding later this year.

They are not a celebrity couple. It is a conscious choice, she says. And easily done. They avoid the premieres and the paparazzi. They just spend their time at home. You're nesting, I suggest. ''Yes,'' she says and then quickly adds, ''I'm not expecting, though.''

How has she changed in the last eight years? I ask her. ''I haven't got any taller,'' she says, smiling as she avoids the question. She smiles a lot. There is plenty to smile about.

Our time is up. There's a car waiting for her around at the hotel door. After our chat she has the rest of the day free. She is off to have lunch with her friend Claudia. Maybe she'll take in a movie. Or just go for a wander around London. She is in no hurry to decide. No hurry at all.

State of Play starts on BBC1 on May 18 at 9pm.