Never enough for Duff

'I'll always be known as Shameless star Anne-Marie Duff,' laughs the actress in a rehearsal room overlooking the Thames at the National Theatre. 'In fact, everyone who has ever been in Shameless is always referred to as "Shameless star".

We could run through Buckingham Palace starkers and we'd still be known as that actor from Shameless. There's no possibility of reinvention.'

Which is odd, because reinvention is exactly what Duff does best. She has a very English, girl-next-door prettiness, yet the roles on her CV have rarely played to this quality.

Unbeautiful women under pressure is her speciality - from fag-happy Fiona Gallagher, in charge of her unruly siblings on a Manchester estate, to Elizabeth I in The Virgin Queen, for which Duff had her head shaved. She's played an abused Irish teenager in The Magdalene Sisters and a mother on the run from a violent partner in Dominic Savage's TV drama Born Equal.

She tends to play women who are both tough and vulnerable and, on one level, extreme. 'There are some roles you just don't say no to,' she says. 'Those compass points: you get them so rarely as an actress.'

Her new role is a case in point: Joan of Arc in Marianne Elliot's production of George Bernard Shaw's 1924 play about the 19-year-old devout French Catholic who led the triumphant campaign to defeat the English at the siege of Orleans in 1429, and was later burned at the stake for heresy.

Duff, now 36, describes it as the part of a lifetime and says that subconsciously she'd always wanted it. 'When I got the call, this bell went off; it was as though I'd been waiting for this since I was 18,' she says.

'Joan is a real firebrand. She has this huge commitment to God that these days we associate with fundamentalism. It's a really interesting play to be putting on now: it's about land rights, and the whole world is fighting over that at the moment. It's easy to see Joan as a young Palestinian.'

Youthful parts are a Duff forte - she frequently plays women a good decade or more younger than her own age and says she loves accessing the unquestioning, fearless energy of youth that evaporates as you get older.

These days, she lives in what appears to be blissful stability with her husband, actor James McAvoy, whom she met on Shameless, and whom, I am warned beforehand, she will not talk about. However, this reticence seems born more from a desire to remain resolutely unstarry than from actorly preciousness, perhaps the product of her working-class upbringing in Hayes as the daughter of Irish immigrants who came to London in the 1960s.

'When I started drama school, theatre was the main draw. I never had any movie star notions. Not that there were family ties to the theatre, either. I only went along to youth theatre with a friend when I was young, to try to make myself a bit more sociable. But the whole thing was quite sore; it really hurt me trying to get into drama school.

'It was a world I knew nothing about - it was very middle class; all that usual stuff. But I was young, determined and I just went for it.'