Ask most Hollywood actresses why they took a part and you’ll get some mush about scripts and heavenly co-stars. Ask Kelly Macdonald why she chose Skellig, Sky’s adaptation of David Almond’s classic children’s novel, as her comeback role after giving birth to a son last year, and you get a real answer: “I couldn’t believe my luck because I was a little concerned about how I was going to shape up after having a baby. And then this came up. It was just this really charming little film. And the fact that my character, Louise, is pregnant at the beginning, and then has a baby, meant I didn’t have to go to the gym. Everyone’s a winner!”

Not the sort of admission you’d extract from Angelina Jolie, but then Macdonald is a very different kind of actress. Shy, self-doubting and candid to a degree that makes publicists want to defenestrate their BlackBerries, she has an honesty that makes her intensely likeable, and which also characterises her work. “I’m not a scene-stealer in any way. I work on what I think is the most believable thing — which often isn’t the most memorable thing,” she says. That might be one reason why her name has never been the first one on the posters. Even so, the world’s top directors know exactly who she is.

Since audiences first saw her face, and most of the rest of her, naked atop Ewan McGregor in Trainspotting, Macdonald, 33, has appeared in a steady succession of memorable supporting roles — Paul Abbott’s TV drama State of Play, Robert Altman’s Gosford Park, an Emmy-winning performance in Richard Curtis’s The Girl in the Café. But it was her performance as Josh Brolin’s wife in the Coen Brothers’ No Country for Old Men in 2007 that brought her career new momentum.

Typically, she demurs. “I don’t think me and momentum go together. It's never been that. But I think certainly as far as the US is concerned I’ve kind of been tried and tested, so in that sense, yeah, there’s more happening.”

“More happening” is a Macdonald euphemism for her casting in Boardwalk Empire, the new HBO series about Atlantic City in the 1920s from Terence Winter, a lead writer on the The Sopranos. The pilot is to be directed by Martin Scorsese, in his first outing on television. It will be the hottest thing on TV, and Macdonald, who plays an Irish immigrant who marries to get out of her parents’ home, goes to film the pilot in the States this summer.

Add Scorsese to Altman and the Coens and you are somewhere near a full house in Great Contemporary Director Top Trumps. Just don’t expect Macdonald to start boasting.

“I don’t know. I could always do better. I’ve done so many dowdies! Even the ones that aren’t dowdy on the page I somehow make them dowdy on screen. I would go as far as to say dumpy.”

Time to glam it up a bit, perhaps? “I suppose so. I want to be able to do make-up and no make-up. Start with that, then I’m happy.”

Name your favourite . . .

— Onscreen Mums

Clare Skinner in Outnumbered
I’ve been watching it recently and she’s brilliant in it. I’m so pleased she’s been nominated for a Bafta.

Glynis Johns in Mary Poppins
I know that Mrs Banks [Glynis Johns] isn’t a very good Mum in it — hence the need for Mary Poppins — but she’s just always really tickled me. She’s so passionate about the suffragettes and I just loved her singing voice. She does love her children — she’s just a bit distracted.

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