Despite wowing critics with her performance as a prostitute in the little- seen Stella Does Tricks, MacDonald has remained off-screen ever since, while the likes of Ewan McGregor and Robert Carlyle have begun to register on the Hollywood consciousness. This is set to change, however. MacDonald is about to become ubiquitous.
Featuring within the space of a month in two high-profile period dramas - Cousin Bette, followed by Elizabeth - MacDonald mania will then truly begin as four recently completed pictures are released. Gregg Araki's Splendor, Mike Figgis's The Loss of Sexual Innocence (alongside fellow rising Brit Jonathan Rhys-Meyers), Hugh Hudson's My Life So Far and Entropy with Stephen Dorff, should go some way to ensure that she usurps Parker Posey as the queen of independent cinema.
"It's been completely my choice. I've not really been bullied into anything I didn't want to do," says the 22 year-old, resolutely defending her eclectic range of choices. "I've been lucky. Every single thing I've done, I've learnt something from for different reasons. I've not got a pattern to it all, I've just been trying out different things."
I ask her what she learnt on Cousin Bette, a kind of Dangerous Liaisons without the venom. I receive the innocently earnest reply: "How to ride a horse, and how to get out of a corset myself."
It's hard not to recall in moments like these her pre-Trainspotting cannabis experience: she was sick in a Glasgow creperie, and awoke from a really nice dream about a princess. Can anyone really be this sweet?
She appears tiny in her Dorchester suite. She swings her legs under her chair and giggles continuously through the interview, reaching a point of hysteria as she realises the word "sets" sounds like sex.
"Before Trainspotting, I was quite awkward in company, and shy," she admits, as if to qualify her nerves. "I would either not say a word, or babble like a lunatic and not make sense. I can now take my time over what I'm saying. But I don't think Trainspotting has made me into anything I wasn't before, or I wasn't going to be anyway."
In Cousin Bette, based on the Balzac novel and directed by the American playwright, Des McAnuff, MacDonald plays Hortense, niece to Jessica Lange's calculating Belle, but barely has the chance to stretch herself. She spends much of her time - with a faultless English accent disguising her thick Glaswegian brogue - sobbing into her handkerchief.
"I wanted to prove to me that I could do something else. I wanted to get away from the 16-year-old, contemporary, sexually-active young girls," says MacDonald. "Hortense is a wee bit older. She's not a bad person, but quite spoilt, naive and has very romantic ideas about love and life. She's quite hysterical, really."
It's a performance to be praised technically, if not emotionally. And the same could be of MacDonald's turn in Elizabeth, as fine lady-in-waiting to Cate Blanchett's Queen Elizabeth. Ever ready to pop the stardom bubble, MacDonald admits: "There wasn't a lot of work in it. I was just standing there behind the queen".
The glamour of the industry, though, continues to fascinate: "With Cousin Bette, I couldn't quite believe I was working with these people. I kept staring. I can't help it, I just get star-struck. When Jessica was on stage in London doing A Streetcar Named Desire, I went to see the show, and I went to say "hello" afterwards. I'd got it into my head that she wouldn't remember me, which was ridiculous as I'd spent two months with her. I was still really excited when she saw me and gave me a hug."
A recently installed resident of Old Street in London, MacDonald still spends much of her time flying back to Glasgow to visit her folks (father's a painter and decorator, mother's a stress counsellor - "which should come in handy, though I don't take any notice of her advice"). It was here that she won her role in Trainspotting as the schoolgirl seductress. Despite a lack of formal training, merely a brief spell in an amateur dramatics club, MacDonald went to the auditions (without telling anyone) just to see what they were like.
"If Trainspotting hadn't happened I would've eventually plucked up the courage to audition for drama school, spent three years there and God knows how long trying to land a role. It was a very privileged way to get in the industry. It was just a bizarre thing to happen. I remember reading about things like that in teen magazines, and thinking it doesn't really happen like that. And then it happened to me."
Or not, as the case may be. Missing out on the Cannes experience that sealed the film's reputation, MacDonald's infamous sex scene with Ewan McGregor was also trimmed in the States because she appeared to be having too much fun, censors felt.
"There was such a buzz about the film," she remembers, "but people didn't recognise me. I could stand next to the poster and people wouldn't bat an eye."
Uncertain of her next project, MacDonald has taken the opportunity to increase her profile further. Appearing at the recent Edinburgh Festival, she participated in the first live reading of a psychological drama called Dark Blood by Fiona Watson.
This was a reaction - like her run at the Old Vic in Hurly Burly last year - to the mundanity of film-making. "I think there must be more to it than smoking cigarettes and drinking coffee," she muses, as if looking for an answer from me.
Less hyped than the Land Girls trio of Rachel Weisz, Catherine McCormack and Anna Friel, MacDonald is more of an original, her uncertainties leading me to believe there's no front. "It does feel like I'm doing what I'm supposed to be doing now. I don't know how long it's going to last, but it feels right at the moment."
Cousin Bette opens next Friday. Elizabeth is released on 2 October.