Angels and demons

"The thing is," she says, "I had no empathy with this woman at all. We just don't have the same background." She's talking about Anna, the vulnerable-yet-tough, slightly kooky single mum on the lookout for a nice bloke she plays in The Waiting Room, a romantic film that opens tomorrow. What's the problem? "I just don't do middle-class angst." Until now, this has been true. Duff was born to Irish parents in a council estate in Hayes, London. Moreover, the 37-year-old has specialised in playing characters who, if they have been at all angsty, certainly have not lived in an Edwardian terraced house in a well-to-do area, nor decorously flirted with a hunky therapist at a squirmworthy dinner party, as her character does in the film.

No: Duff is known for playing women of sterner stuff. Her breakthrough performance was as King Lear's chasteningly honest daughter Cordelia in the National theatre's 1997 production, opposite Ian Holm. She played an abused teenager (despite being 30 at the time) in Peter Mullan's harrowing 2002 film The Magdalene Sisters. In the TV drama Born Equal, she played a wife on the run from an abusive partner with a small kid. And last year, she won an Evening Standard best actress award for performance as Saint Joan in the National's revival of Shaw's masterpiece.

Most pertinently, she starred for 18 memorable episodes in Channel 4's often brilliant Shameless. Wearing a uniform of pink velour tracksuit and oversized earrings, she played Fiona Gallagher, a woman who holds together a family headed by an alcoholic, while also finding time to go out with sexy bad lad Steve - played by James McAvoy, who would later become Duff's real-life husband.

So why climb into Anna's shoes if you can't empathise with her? "She's complicated. She's not just a bunch of cliches, not just a flustered single mum, not just a Bridget Jones. There are certain stories that grab you by the throat and hers did. She doesn't know how to put her life back together. Any kind of conflict draws me to a role."

With its sumptuous cinematography creating an implausibly picture postcard London, The Waiting Room could easily have been one of those set-in-the-capital misfires, like Woody Allen's Match Point. But, thanks in great measure to Duff's performance, it manages to be something less risible.

Variety has called the film "basically a meet-cute yarn". It's a description that has Duff once again pinching her eyebrows. "It's more than that. It's about those moments when you glimpse someone, perhaps as a car door opens, and it becomes freighted with a huge significance. You want them to get together. You want them to experience joy." She gives a glowingly upbeat smile.

Duff says she now plans to retire from the limelight for a while: "There are times when you have to get away because people are sick of the sight of you." Are people sick of you? "If you do an iconic performance, say, of Hamlet, you'll find it hard to get work afterwards. I haven't had an offer to work in the theatre since Saint Joan. I can see why: I'm so identified with the role that they think, 'Not her again.'" You almost sound as if you regret taking the role? "Jesus God, no! It was the most amazing summer of my life. I wanted to become an actress when I was 17 for roles like this. I know this sounds pretty wanky, but waiting in the wings to go on at the Olivier was just like waiting to play Glastonbury."

Wasn't it a risk to take on an unfashionable heroine? After all, these days Shaw is widely (if wrongly) taken to be a didactic windbag. "That's why the rewards were so mighty. I thought I'd never be a darling in terms of classical theatre." But now this classical theatre darling is taking a break, having completed a clutch of films that are now looming: there's French Kiss, a rom-com starring her and Eric Cantona ("I play a nice sweet girl. Obviously a challenge"); and Is There Anybody There?, a British drama set in an old people's home. But I'd read that her husband was taking a break, not her. Under the headline "McAvoy will quit movies for wife", he was quoted as saying: "I'm slowing down now and I'll be her bitch while she makes a movie."

"I suspect that's bollocks," says Duff, "but I don't really want to comment." Fair enough. Anyway, she points out, the couple have just returned from filming in Wittenberg, Germany, where they worked together for the first time since Shameless. In The Last Station, an adaptation of Jay Parini's novel about the final year of Tolstoy's life, Duff plays the writer's daughter Sonya and McAvoy his amanuensis Bulgakov.

Neither of those characters attracts Duff's attention like the long-suffering Mrs Tolstoy: "I identified with her character absolutely. I don't know what it's like for you, but there are certain times when you go into a room with your partner feeling like a failure for being alive. She had her problems, but try living with Mr Perfect." Disappointingly, Duff is talking about the marriage of Sophie and Leo Tolstoy, and not her relationship with McAvoy, the 29-year-old Bafta-winning, limelight-grabbing Glaswegian star of The Last King of Scotland and Atonement.

The couple might not have met on the set of Shameless were it not for Duff's mother. "I remember I was unsure about doing Shameless. I'd never acted in anything so commercial. I read the script in the garden with my mum, Mary." What did she make of it? "She said it's filthy dirty, but she said these people have love and sex and nothing else. That made me take the role."

Does she ever feel envious of McAvoy? "Yes! He's a real film actor, in that he really knows how to work a film set in a way that I'm still learning. In theatre, there's the director, the writer, and below them the actor. In film, it's the actors who are most important. That goes against the grain for me. It's been amazing for me to see the self-confidence of actors who insist on having control, because it's going to be their faces 20ft high in the posters.

"I've been shocked by film actors - 25 and under - having such confidence and cockiness to rewrite a scene. My background is more about the director being in control. It's all about yielding. It's an oddly submissive relationship in which you're moulded, Pygmalion-style." Surely you're more than Pygmalion? "I want to be. As I get older, I want to draw on my experience to make roles better. I see that in the older women who inspire me - their experience makes them better."

Who specifically? "Well, I hope I have as many great roles in my career as Judi Dench has had since she was 40. There are other women I'm inspired by: Meryl Streep, Kirsty Wark, Jenni Murray. And I can't say Penelope Wilton without a tingle going down my spine." Me neither. But why are you thinking about older role models? "Because everybody's obsessed with freezing their faces. Which is a bit of a shame. You get better as you get older. That's what I plan to do."

The Waiting Room is out on Friday