There's something a bit Kylie-esque about Kelly Macdonald: she's so small and perfectly formed, and when I see her first, dressed in a glittery frock, with her hair and make-up done, her skin dusted with a golden shimmer, she's so sparkly and shiny she reminds me of nothing so much as a Christmas bauble.
Ten minutes later, she reappears in her own gear, a pair of jeans and a masculine, oversized, checked shirt and practically disappears into the wallpaper. Though she's taken pivotal roles in some of the best films of the past decade - Trainspotting, Gosford Park, No Country For Old Men - she's weirdly unrecognisable. Or, not unrecognisable, perhaps, just somehow unremarkable. I study the waiter when he comes to take our order, but there's not even a flicker of recognition.
But then, a less actressy actress it would be hard to find. She's so charming and polite, but not in the standard I-must-be-charming-and-polite-towards-the-lower-orders sort of way that most actresses are when interviewed by journalists. It just seems to be the way she is: diffident, unaffected, wholly unstarry.
Her life has changed beyond recognition since she attended an open audition for the part of Renton's schoolgirl one-night stand in Trainspotting, but it's hard to imagine that she has personalitywise, although she says she's much more confident. But that's not saying much. Reading her cuttings, the most frequent comparison made is to a terrified small child.
"It's just a character flaw. It's just the way I'm made. I do feel confident in what I do. It doesn't always work out 100% of the time, but generally I think I can do it quite well. But the other part of my job is doing the press and stuff. And I'm rubbish at that. I'm really not good at that at all - this quite important part of what I do."
You're not rubbish, I say.
"That's... yeah... you're very kind... but I'm just not... I'm not in my comfort zone."
It's true, she's not in her comfort zone. Most actors deign to answer your boring questions. If you're lucky. But Macdonald approaches them like it's a job interview and her next pay cheque hangs in the balance.
But then perhaps it's partly because this is the first round of publicity since she had a baby last year. Freddie, her son with Dougie Payne, the bassist from the Scottish band Travis, was born last March, and Skellig, a fantasy drama for Sky based on the award-winning children's book by David Almond, is the first piece of work since.
It's a very expensive-looking quality drama of the sort that you don't quite expect to see in these recessionary times. But then, although it's being broadcast here on Sky, it's getting a cinema release overseas and there's an all-star cast: Tim Roth plays the eponymous Skellig, Bill Milner from Son of Rambow is the lead character, and John Simm and Macdonald play his parents, the only duff note being Alexander Armstrong as the wholly improbable Geordie PE teacher.
Having played journalistic colleagues in Paul Abbott's State of Play, one of the best TV dramas of recent years, there's something completely natural about seeing Macdonald and Simm playing man and wife. "His character is off doing all these other things in State of Play, we were just the business end of things, so it's quite nice to go home with him. For it just to be us inside a room and for it to be our story. We're in this little kitchen-sink drama within the film... there's all this magical stuff going on elsewhere which we have no part of."
Annabel Jankel, the director, said that Macdonald's name was mentioned in the early stages. "Everybody latched on to her as the absolutely perfect person to be Louise, and we were beyond thrilled when she said yes. She has this incredible mixture of vulnerability and strength in equal measure."
Skellig, Macdonald says, is an "ideal" film for her post-Freddie's birth: "It was just the right thing at the right time. The character is pregnant and then has a baby, so I didn't have that mad pressure to lose weight and it just felt very comfortable. Or as comfortable as you can be in a prosthetic belly. I had two of them, a prosthetic pregnant belly and a post-baby one, although I didn't really need it. One of the clothing assistants came along and patted my stomach and said, 'Oh you've your post-baby belly on.' And I was like, 'Well, yeah, but it's my actual one.'"
If Skellig was Macdonald easing herself back into the world of work, her next project is likely to be total bodyshock. She's been cast in a leading role in one of the most heavily anticipated projects of the year: Martin Scorsese's Boardwalk Empire. It's set in the 1920s and is about an Irish family in Atlantic City, and how it became a centre of gambling. At the moment it's just a pilot for HBO, but its ambitions are evident not just from the choice of director but from the fact that the script is by Terence Winter, who, after David Chase, was chief writer on The Sopranos, and it stars Steve Buscemi, an ex-Sopranos actor.
It's Scorsese's first TV drama since 1986 and there are high, high hopes that it will become a long-running serial and fill the void left by The Sopranos. According to Variety, Macdonald is to play "Margaret, a smart Irish immigrant who married the wrong man to get out of her parents' house", but when I ask her how she came to land the role, she just shrugs confusedly. "I really have no idea. These things just sort of turn up. I just get a phone call. It's always a bit worrying when you haven't had to slog your way through the audition process and it just turns up out of the blue."
She has met Scorsese though, having auditioned for him years ago, and if the pilot goes well it could take up the next eight years.
Are you prepared for that?
"I have to be. That's what you sign for."
So would that mean relocating?
"For periods of time. I might feel differently about it if it was the west coast of America, but the fact that it's New York is appealing. It's closer to home and it's an easier way of life. LA is brilliant, but however long my trip is I'm always ready to leave. But New York I'm never quite happy to see the back of. And also, having a baby, if we're going to live somewhere else for a wee while, it's quite a good time for us."
You don't have to worry about schools and things, I say. "You don't. People do, but I'm not one of them."
It's also likely to turn her into the star she's never quite become. Her role as Carla Jean Moss in No Country for Old Men has exposed her to an American audience like never before, but even having a leading role in one of the highest profile films of last year, she still somehow managed to disappear into the film.
But in a way this is one of her key strengths. Clark Gregg, who directed her in the film of Chuck Palahniuk's novel Choke, refers to it as "invisible acting". He describes her as having a "volcanic reserve of silent strength", while Colin Firth, an old friend, says that "she can pop up and be dazzling or fade into the wallpaper as required. She has real presence."
I'd read somewhere that the Coen brothers had resisted even seeing her for the part, thought their casting agent was quite mad to even suggest her. They'd wanted to keep the film as local as possible to west Texas, where it was being filmed, and Macdonald had had to lobby for the part. But she's almost offended by the idea.
"No, I didn't lobby at all. I don't lobby where work is concerned. As long as I know I've got something coming up, I don't really worry. It's not that I'm not ambitious, but I don't have a drive to be hugely successful and be working all the time. I was just in New York for a wedding. And my agent thought it would be a good idea to meet the casting agent. It was really early days and then that turned into her putting me on to Joel and Ethan even though they weren't casting at that time. They saw me and it obviously went well because I got the part. But they did see absolutely everyone else. And they really held out for a Texan. I think it was kind of annoying. Joel said as much. He was determined that she was going to be a local. And then they ended up with me."
She went into the audition chatting away in her usual Glaswegian tones and flipped into a west Texas accent that seems to have left the Coen brothers gobsmacked. Josh Brolin, who played her husband, said she had the strongest accent he'd ever heard. "I couldn't understand her literally 70% of the time and then she would be just flawless with her Texas accent."
But she's always been good with accents. She peppers her conversation with funny voices and when she tells me what Anthony Hopkins once told her - "If you want to make it in this town, you need to talk loudly and chew gum" - she does it in a marvellous Welsh baritone.
Working with the Coen brothers was a dream come true. "It was one of the best experiences of my working life. It was just so easy. I didn't have to worry. You could place your trust completely in them. They just knew exactly what they were after. They do all the work."
It seems amazing that even after being in one of the most highly acclaimed films of last year Macdonald has managed to avoid the limelight. She doesn't do the celebrity magazine thing. She says she never gets papped. And when I look up pictures of her on the internet, one of the few I find of her with Dougie is on Flickr, taken by a fan who's bumped into them on the street and asked them to pose, which they do, smiling sweetly. It's hard to imagine Gwyneth Paltrow and Chris Martin being quite so amenable.
It makes you realise that being a celebrity is a choice. And it's one Macdonald and her husband have refused to make. They live in north London, go back to Glasgow regularly, and hang out with the friends they made before they were famous. They're like ordinary people. Just richer.
But then, perhaps, what's saved Macdonald, and what marks her out as an actress, is that she didn't go to stage school or drama college. She didn't star in terrible student productions, or have bit parts in Casualty. She acts but never really became an "actress". She was simply an ordinary girl. Who got a part in a film: Trainspotting, the Slumdog Millionaire of the mid-90s.
"It's just mad when you think about it. I watch all those X Factor type programmes and my audition was kind of like that. It's like I won The X Factor, you know? And without having cameras in my face and everyone knowing my business."
Maybe it's this "knowing your business" side of the deal that has kept Macdonald in relative anonymity. She grew up in Glasgow's Southside, a comfortable, middle-class childhood, until the age of nine, at which point her father, Archie, a painter and decorator, and her mother, Patsy, got divorced. There followed a succession of schools and temporary homes, until finally Macdonald, her mother and her brother were rehoused by the council on an estate in Newton Mearns.
She's reluctant to elaborate on this. "It's not very interesting. It was fairly middling middle-class up to a point and then money struggles hit. We didn't starve or anything. It just meant... Dougie went on quite exotic school holidays to France and things like that, which we obviously didn't. But my mum always looked after us."
It's her father she had problems with. They fell out in her teens and she no longer has any contact with him. He gave an interview to a tabloid a few years back and blamed variously her mother and her new-found celebrity, but the whole thing is clouded in uncertainty as Macdonald has steadfastly refused to speak about it.
Does being famous add another level of complexity to family relationships?
"It doesn't make it more complex. It just means that more people know your business. And if you're not in the business of selling your business..." And she trails off into mid-air.
School wasn't something she particularly enjoyed and she left at 16, had a brief spell in college, and was working as a barmaid when she saw a flyer for an open audition for the part in Trainspotting. Hundreds of girls turned up.
"It was in a huge hall at the University of Strathclyde. And Danny [Boyle, the director] was at the end of the room at a table. It really was just like The X Factor. The first audition was just chat. What do you do? What do you want to do. Everybody would move forward a row and get closer to the front. And I remember when I was in the second row, Danny looked at me, and I kind of had to look away first."
Boyle puts it baldly: "It was a terrible cliché, but as soon as she sat down I said to my assistant location manager, 'That's her.' I think Kelly has that thing Ewan McGregor has, indefinable star quality, yet they're ordinary people."
It was a life-transforming moment for Macdonald. A decade or so on, if you say to anyone, "the schoolgirl in Trainspotting", they know exactly who you're talking about. She was brilliant in the part, naive yet confident, and she still gets cast in things today by directors who remember her in the film.
"I don't know what I'd be doing if I hadn't got it. Theatre, perhaps. It kind of feels like this is where I was meant to be. I'm on my right path. Not in a spiritual way. I can't really imagine it happening any other way. If I'm honest, it feels like it's destiny. There's a Scottish entrepreneur, Tom Hunter, and I was with Richard Curtis at something and Tom was there and we got talking, and he's got this theory that you know from when you're tiny if you're going to live a life that is enriching in some way, or out of the ordinary. So, over the years, he has asked people, did you know? And everybody knew. And I feel that way, I always felt that there was more out there. And where I lived that was really unusual."
I happen to believe it's the other way around. That all the people who get sent home from The X Factor think that, too, but we all make up the story of our life, as we go along, from the facts at hand, and the facts, in Macdonald's case, are that she does seem to have been sped to the land called success by the guiding hand of destiny.
In anybody else, you'd probably hate her for it. But you can't hate Kelly Macdonald. It would be like hating kittens or strawberries. She heads off to pick up Freddie from the childminder's and I can't help hoping, for her sake, that Boardwalk Empire isn't too big a hit.