'I prefered kissing Bill Nighy'

"I loved doing the film," says Macdonald. "We shot it in four weeks just after Christmas. I really wanted to do it because I've always loved Richard's writing and I've worked with the director, David Yates, and the producer, Hilary Bevan Jones, before on State of Play. Bill [Nighy] was also in State of Play but we didn't get many scenes together, so it was wonderful to have this intense performance with him."

But is a love affair between a washed-up wonk and a fresh-faced Scots girl a bit farfetched? "Oh, I don't think so!" she says. "Any woman who met Bill would fall in love with him!"

Interestingly, though, she didn't want to be in the film for political reasons. When I ask her if she is going to Live8 she looks nervous again and says, "I don't know when it is, actually." She doesn't think she's going to march on Edinburgh, either. "Oh, I'm not political," she says. "I'm an actress!

"People ask me about the politics behind the film but I am just interpreting what Richard has come up with. The words are his, not mine. My job is to dramatise it. Gina is the voice of the average person who wants to ask the obvious questions politicians seem to ignore. It wasn't the love scenes I found difficult but the scenes when I have these long speeches, where I am questioning the world's leaders. They were much more difficult than kissing Bill!"

Macdonald has been through the experience of being recognised. Ten years ago when she was 19, she was plucked out of the blue from her "average life, growing up in Glasgow", as she describes it, to play Diane, Ewan McGregor's girlfriend, in Trainspotting. Macdonald then became the epitome of cool and her image was turned into a poster that adorned many a boy's wall.

Even Eve McGregor, Ewan's wife who was pregnant with the couple's second child at the time, said that Macdonald is the only co-star of her husband's that she has ever felt threatened by. Macdonald always says she finds that amazing. "He's starred with Nicole Kidman and Natalie Portman!" she says with a bit of a why-on-earth-would-she-be-worried-about-me tone to her voice.

She says she spends a lot of time not working. She often won't go for a role because she thinks she'll be no good at it. Since Trainspotting, she says, acting jobs have been erratic, even though she's been in Elizabeth, Gosford Park and the acclaimed TV drama, State of Play.

"After Trainspotting I even went back to the pub in Glasgow that I was working in before," she says. "Then I spent some time being asked to do all those photo shoots for 'upandcoming actress' lists, that type of thing. But now I'm 29 I'm not asked to do them any more. Maybe it's just too embarrassing. I mean, I'm not a trained actress. I got the part of Diane because of an open audition. For years I just thought I was a pretend actress, really."

The truth is, attention makes her feel uncomfortable. When she and Payne got married on the shores of Loch Fyne in Argyllshire, the Scottish press were dogging their every move.

"I hated that," she says. "I like to be anonymous. We don't get attention here in London. We go to the pub in Kentish Town near where we live and we see bands and go out for a meal. We don't get snapped, but the Scottish papers wanted pictures of the wedding so we came out of the reception and did a few and then went on to have the best day of our life."

Has marriage changed her? "Not really," she says. "We've been together five years and he proposed two years ago on top of the Empire State Building, so we've had a long time to get used to each other."

Her parents' marriage fractured when she was nine years old, so she and her younger brother, David, who still lives in Glasgow, were then brought up solely by their mother, Patsy. "She worked in an office in Glasgow to make ends meet," says Macdonald. "I don't ever really think that my dad left us, more that we left him, really. But it's funny now because I didn't do that well at school. I was always convinced that I'd never amount to much.

"I loved drama but I kept it secret. I used to hole up in my room and record myself reciting speeches from plays. I tried out different accents. My mum says to people, 'Oh, I always knew Kelly would be an actress', so either she's got second sight or she was more aware of what I was doing than I knew at the time."

Does she not consider herself an actress now? "Finally, I think I do," she says. But not a star? "Oh no," she says, looking appalled. "I don't even know what my next job is. I often go to Los Angeles to see friends and I call my agent there to say I'm coming over and then I have to trawl around meeting people who have no interest in meeting me at all, and it all seems a bit silly."

Then she's off like a rabbit let out of the headlights.

The Girl in the Café is on 25 June on BBC1 at 9.10pm.