Kelly Macdonald: Upstairs and downstairs with Kelly

Kelly Macdonald sits on the edge of the sofa, elbows on knees, chin resting in cupped hands. The sleeves of her moss-green jumper are stretched over her palms. It's an early call on a bitterly cold, late-December morning and she smiles sweetly, if a little sleepily, when we shake hands.

It may be seven years since her notorious schoolgirl seduction of Ewan McGregor in Trainspotting, but Macdonald is still a mere 25 years of age and could easily pass for 19. Since turning up on the spur of the moment for an open casting for Danny Boyle's Nineties classic and thus effecting her own rescue from a dead-end future behind the bars of Glasgow, Macdonald has worked steadily. Mostly the parts have been small, though well received, and she gained good reviews for her lead role in Coky Giedroyc's Stella Does Tricks. In her new film, Robert Altman's Gosford Park, she stars with the great and the good of the British acting fraternity, and holds her own admirably.

As we talk about working with Altman, Macdonald speaks slowly, as if relishing the opportunity to casually refer to "Bob". She's very precise in her speech, though she can't help punctuating her sentences with giggles – sometimes self-deprecating, occasionally nervous, always explosive. They seem to bubble up from her diaphragm and burst out in a great rush, enveloping whatever she's trying to say in a sound best (if not exactly flatteringly) described as being like a foghorn.

Though surprisingly noisy, these moments are entirely genuine, part and parcel of Macdonald's natural lack of irony or cynicism. She isn't careful about what she says because she has no axe to grind. On Altman, for example: "The first film of his I saw was Short Cuts," she tells me. "I was about 16 and had gone to the cinema with a friend to see something else which was sold out, and the only other thing on was this film I had never heard of called Short Cuts."

She's caught up a bit since: "I've seen MASH, Dr T And The Women, Cookie's Fortune and The Long Goodbye, but Nashville's the one everyone goes on about and that's the one I really want to see."

For his part, Altman hadn't seen any of Macdonald's flicks: "He did see the Hugh Hudson film [My Life So Far] I was in," she remembers. "But not until after I had the part, and then he told me that I didn't have anything to do in it and I was like," the foghorn bellows, "'Yeah, I know, it was quite a small part,' and that was it and he walked away." She leans back into the corner of the sofa, pulls on her cuffs and adds, smiling: "So it wasn't my body of work he was impressed with."

Altman didn't formally audition his cast: "It was just him chatting, really," she shrugs. "And if I was anything to go by, the actors were just sitting there going, 'Aha. Uhur. Oh right. Aha.' And I left thinking I'd done really badly, that he'd wanted to find out about my personality and I just sat there and nodded like a dumb person."

Little did Macdonald know that the ability to watch and listen were just the qualities Altman was after for Mary, the servant girl through whose eyes the audience gains access to the various boudoirs and plot conundrums of his country-house murder mystery. "He kept telling me that I was the audience's tour guide and they would be looking to me for all these things. I was thinking, 'Oh no, I just want to be in the ensemble and disappear.'"

It is to Macdonald's credit that she doesn't simply merge into the scenery: she was sharing the set with a roll call of great actors, any one of whom could happily swamp a scene all by themselves – Maggie Smith, Helen Mirren, Alan Bates, Michael Gambon, Richard E Grant... the list, as they say, goes on.

"That was the great thing about it," enthuses Macdonald. "I got to gawp openly at people like Maggie Smith. Normally you're thinking too hard about what you're meant to be doing to take it all in, but that was my character's job in the film."

An astute analysis of the British class system, Gosford Park is split between the upstairs nobs and the downstairs minions. I wonder whether the cast allegiances were similarly split and Macdonald nods: "You build relationships with the people you see most, and because the upstairs and downstairs were split between a location and a set, whenever we all got together there was a wee bit of that." She pauses before proffering one her noisiest guffaws and adding: "But then downstairs you've got the likes of Alan Bates and Derek Jacobi, who are obviously not going to hang around with the likes of me."

It may not always sound the case, but Macdonald says she's finally become comfortable calling herself an actor: "I am unapologetic about it now, whereas before I couldn't bear to say what I did for a living. My boyfriend [Dougie Payne, from the band Travis] is a musician and he finds it hard, too – passport control in airports is especially bad. There's me with a passport full of weird visas, in every one my hair's a different colour depending on which film it was for, and there's Dougie being asked what he does for a living and saying, 'Well, basically I go Uhhhhuhuuhuh.' He plays bass and sings backing vocals and a lot of his job is to stand in front of the microphone and go 'Uhhhhuhuhhuh'."

Does she plan to head for Hollywood? "I don't know. I think it's a good thing to do, if that's what you want," she says with a shrug. "I think it depends on how ambitious you are. And I am ambitious – I thought I wasn't, but I am. But not in an aggressive way. I like the way things have been going and I have faith in the fact that something else good will come up."

She's yet to learn how to cope with the schmoozing side of the business: "When you're on a big film and you have to have dinner with all the execs, my brain goes into shut down. I'm sitting there at the table, supposed to be making an impression on these people and I get so stressed out, it makes me want to fall asleep."

Luckily, Macdonald found the dining arrangements on Gosford Park less daunting. One of the observations that Altman makes great play of is the self-imposed hierarchy of the downstairs community, a fact made plain by the seating arrangements at meal times, when the maid of the most important visiting aristocrat gains the top spot at table.

Macdonald hugs her knees, remembering the scene: "That was one of my favourite days on the film," she explains. "Because Bob likes to get things done – he doesn't like to linger – it meant that the table had to be lit from everywhere, so the camera could just move about. And so we all were sitting there for ages while the crew busied about us, and it was really good fun – like being back at school again, just chatting.

"Richard E Grant was brilliant," she continues. "He's really nosy and he asks questions all the time, really forthright. He went round the table asking everybody what their nicknames were at school – questions that people were a bit taken aback by – and managed to get everyone to join in."

I ask whose nickname was the best, and Macdonald lets out a little gasp before producing a cheeky grin and whispering conspiratorially: "Derek Jacobi. His was Stinky," and she lets out a gust of laughter loud enough to create havoc in the shipping lanes of the Thames.