For Macdonald it all happened four years ago, when the 19-year-old barmaid auditioned for a new film called Trainspotting. A poster girl for one of the best British movies of the decade, she was soon winning roles opposite everyone from James Bolam to Jessica Lange. Lady-in-waiting to Cate Blanchett's Elizabeth last year, Macdonald now looks set to become the crowned queen of British cinema with six new features.
Riding such a wave of success, you might forgive Macdonald for playing the prima donna but there's no hint of vanity or self-indulgence. Instead she's modest and matey. Too down to earth to pose dreamily for the photographer, she squirms self-consciously in an old jumper. When she raises her voice above a soft Scottish murmur, she worries that the other patrons of Soho House might think she's showing off. (This being a see-and-be-seen private members mecca for soap stars, media types and other professional show offs, the fresh-faced film star has nothing to worry about).
Brought up on a council estate by her mum, Macdonald didn't enjoy school much. English and art were all right, but she was always getting report cards saying: ``Kelly doesn't try unless she's interested, and she must get used to the fact that she can't always be working on something she enjoys."
"Well I am now," she says, with a gurgle of gleeful laughter. Macdonald had her first taste of acting at 16, when she appeared in a local am-dram production. "We did this terrible Greek play," she remembers. "There were five of us on stage and about ten in the audience. None of us had a clue what it was about."
Although she "got a buzz" from being on stage, Macdonald still was not sure what she wanted to do when she left school. "All my friends were going to art college. Part of me felt as though that's where I was supposed to go too. The only trouble was, I was never very good at it." Macdonald moved into a flat with six art students and she began working behind the college bar. "It was fine for a while, but it started really getting to me. I just thought, `This is wrong! I'm not doing what I want to be doing, and I don't know what I want to be doing!'."
Procrastinating in the pub, Macdonald had just got round to "applying for application forms" for drama school when she landed the part of Diane in Trainspotting. Hit on by Ewan McGregor's junkie Renton in a nightclub, Diane drags him home, has her wicked way with him, then waves him off the next morning - wearing her school uniform. Under-age and over-sexed, Macdonald is magnificent, but offscreen she didn't share Diane's cool confidence about jumping into bed with McGregor.
"I was really nervous. Absolutely dreading that scene but I knew that I just had to go for it, as, if I didn't get it right, I was going to have to - I was going to say `bang away at it' - but maybe that's not the best choice of words. Ewan's used to doing sex scenes so he really helped. Then again," she muses, "all he had to do was lie there." Macdonald does not keep in touch with McGregor - "The last time I saw him, I was wandering around lost in Soho and he told me how to get to a pub" - but she has become firm friends with her other co-star, Ewen Bremner, who played the gormless Spud.
"He took me under his wing. I didn't know London. I didn't know anybody down here. So wherever he went, I went." Later, he introduced Kelly to his agent, who agreed to represent her. "The Trainspotting hype was incredible. It lasted for quite a while but I didn't think anything else was going to come of it." At first Kelly was bombarded only with "druggy, clubby" scripts, but soon other roles came trickling in, such as the plush costume drama, Cousin Bette.
"It was really terrifying working with Jessica Lange. The first day of filming was awful. I had this scene with her where I've got my head in her lap and she's wrapping my ringlets ready for bed. I knew all my lines but I just kept fluffing and forgetting, and going blank. Then I kept apologising to her which I think was doing her head in after a while because she was like - gritted teeth, brittle voice, hand on hair beginning to stroke a little harder - `It's fine!' If we had just been walking down a corridor together, or something, it might have been all right, but to be lying, there looking at her upside-down, was a bit bizarre."
It's strange to hear Macdonald talk about nerves when her screen image is so strong. After her tough performance in Trainspotting, Diane's punchy persona lives on, not only in her film roles but in a new commercial that sees Kelly strolling around a boxing gym, recommending a new anti-perspirant as "strong as a woman". "This is about as energetic as I get," says Kelly, "sitting around talking and drinking coffee. When I was asked to do the advert I thought, `Oh dear, I hope they don't want anything too athletic'."
Some time before, Macdonald's confidence had taken a beating at an audition for the sci-fi movie, The Matrix. "I walked into the room and they had a punchbag hanging there and the cameras set up and I thought, `Oh My God!'. I got really self-conscious. And of course they said to me, `We just want to see what you can do because the character's quite feisty, quite energetic.
"They asked me to run up to this bag, and punch it and kick it. But, because it was a small room, I had to run at it in a curve [mimes a crab- like, comedy run up] and the first time I hit the bag I apologised."
Generally, says Macdonald, casting directors mask their feelings well, but this time it was different. "The look on her face was so obvious. Everyone else in the room was silent. This poor actor who was reading with me didn't know where to look. I left thinking, `I don't want to do this anymore. It's a stupid job'."
Happily, Macdonald climbed straight back into the ring. Along with the Tube Tales, a feature-length collection of short films set on London's Underground (which gets its first screening on Sky Premier tomorrow night), forthcoming films include the Welsh bingo comedy, House!, and a screen adaptation of the Royal Court's Some Voices. Next week she's off to start work on Strictly Sinatra, a new drama from writer-director Peter Capaldi about a casino cigarette girl and a "really crappy" pub singer, who is played by Ian Hart.
"Work's been mad this year," she smiles. "But in a good way. I pinch myself every now and then to make sure it's all real."
`Tube Tales', Sky Premier, tomorrow, at 9pm