Anne-Marie Duff: 'I wondered if I should lie about my background'

Anne-Marie Duff holds out her hand – a shy shake. She is wearing what looks like a child's white vest, jeans and no jewellery. There is nothing to give her away, apart from her face. Even her feet are bare – maroon nail varnish excepted. It is a sunny day – warm upstairs at the Jerwood rehearsal space in south London – and we have two reasons to meet. She is about to star in Strange Interlude at the National, a Eugene O'Neill marathon. And the final episode of Channel 4's Shameless (now in its 11th series) is about to be aired, featuring Fiona – peroxide hair and tarty Gypsy earrings to the fore – taking a last stand. Anne-Marie's range is incredible: Elizabeth I, Margot Fonteyn, Saint Joan, Berenice, John Lennon's mum… and now Nina, complicated, ardent, neurotic war widow. "Interlude", incidentally, misleads. Uncut, the drama is four hours long. Anne-Marie tells me it is one of the plays that made Nicholas Hytner want to be a director. Michael Grandage is another of its champions. I run into Simon Godwin, director of this trimmed version for the Lyttelton and wish him good luck and he laughs, as if acknowledging he may need it.

Anne-Marie is one of those actors who is a chameleon and yet, unmistakably, herself. Her face is uncommonly expressive. It has a wicked gaiety but sorrow comes easily, too. "I am not precious about the way I look. Never having been defined as a great beauty makes that easier." How does she control the detail of what is going on in her face? Here is a test – O'Neill's confounding stage direction to Nina from scene two:

"She appears older than in the previous scene, her face is paler and much thinner… In her fight to regain control over her nerves, she has overstriven after the cool and efficient poise, but she is really in a more highly strung, disorganised state than ever, although she is now more capable of suppressing and concealing it. She remains strikingly handsome and her physical appeal is enhanced by her pallor and the mysterious suggestion about her of hidden experience."

Duff erupts into peals of laughter. "He is like a novelist, O'Neill, isn't he? Our characters are all described forensically at the beginning of these huge scenes. With screenplays, you are advised to ignore stage directions." But she is not about to ignore O'Neill. The trick, she explains, is to rise above literalness and "get the smell of it, breathe it in, see if you can exhale it – that is all you can do". Lashings of white makeup may also be needed to acquire Nina's pallor (Anne-Marie looks as though she spent the Bank holiday weekend in the sun). But the real challenge is to become "more yourself as an actor, visiting every corner". At first she felt unequal to the role, almost turned it down, asked herself: "How can I create this panorama of character? It is not just about charting the years. These people have extraordinary colours that you are trying to find every day in rehearsal. The fluidity of O'Neill's writing is like the ocean – he is obsessed with the sea – it seems to swell in his characters."

Duff almost turned down Shameless, too. This seems to be typical of her. She is careful and carefree – such a mixture. It was her mother who swung it, in the end, saying Paul Abbott's script rang true because "it is about what people have when they don't have anything except laughter, sex and the stars". Or, as Shameless's Frank rephrases it, addressing us from jail: "It is the boredom that gets you. You miss the simple things – drugs and alcohol…"

Duff grew up in Hayes, west London. Her parents are from rural Donegal. Her father was a painter and decorator, her mother worked in a shoe shop. "They taught me many things. Most of all that it is vital in life just to turn up." How does she mean? "To turn up for people, to be present, to have the conversation. This has emboldened me, given me greater empathy." As she says this, her arms are folded and she has a resigned look – sad but in a funny way. She hopes, in time, to pass this "great lesson" on to her three-year-old son. His name is Brendan – after her father.

It is a subject that tends to be treated as if it were a non-issue but I want to know whether it has been difficult to move from one class to another? "My parents were of a generation of Irish people who came here because there was nothing there – out of necessity they came to England. They weren't able to fulfil their dreams." She describes growing up with no sense of "entitlement" – in one sense an advantage. "I knew if I wanted to do this for a living, I really had to pursue it. When I was auditioning for drama schools, the girls around me were from very different backgrounds. I remember thinking, 'Should I lie about my family?'"

She is 42 now, and looking back sees "a tomboy – androgynous until I was 19. Desperately shy. The only confidence I had was in drama." She had been at a comprehensive school in the 80s, "deeply affected by the state of the country: kids weren't interested in politics or the arts". And she adds: "If you had questioned me about anything to do with boys… I was a virgin when I went to drama school [the Drama Centre in north London]. I was naive with the self-righteousness of youth." What about? "It's biological, isn't it? You think you know everything."

The Drama Centre was run by Christopher Fettes and Yat Malmgren – "brilliant but terrifying men". It was a "masochistic" but "exciting" time. "It put me through my paces. I toughened up. I was by no means the star of the year. It taught me to be resourceful, to go away and do the work myself. Invaluable." She points out that it is easier to become an actor from a working-class background than, say, a barrister because "acting is a sublimely egalitarian world". What's more: "I don't feel I've left my parents behind because of the sort of family I come from."

It helps to be married to James McAvoy. They met on Shameless's council estate 10 years ago (he played Steve – he and Fiona had to make passionate love against a Formica worktop). But he comes from a similar family: "Very encouraging and working class." As a result, neither of them has had to suffer "anger or confusion". But it is not a non-issue for her and sometimes it is a joke. She recalls a funny conversation with actor Robert Carlyle, also from a working-class background: "We were laughing, the two of us, saying, 'Just think: our children know what Parma ham is.'"

Anne-Marie Duff has a keen critical intelligence – plenty of sense and sensibility. But she has learned to be careful what she says – and reads – about herself. She avoids reviews: "It is better not to look at them; it is like reading someone's diary. What you think about me is none of my business. It's important to keep faith in the project you are working on." And then there is the question of what she says in public about herself. "My husband has an extraordinary ability to receive a lot of exposure and still maintain a sense of self without giving anything away. I think it's very powerful."

She talks eloquently about how "judgmental" our culture is, deploring its casual cruelty – especially online. She wonders: "How on earth do you teach your child not to be spiteful in the playground when online you can say whatever you like?" She is "no fan" of celebrity culture. She talks, too, about autograph hunters, commenting that it is a racket. (I check later: a photo signed by Duff – as Fiona – can be bought for £24 on eBay).

We talk about the insecurity of her profession. "It comes in waves. I bumped into Olivia Colman recently. She's riding high – she's a brilliant actor and one of the nicest people I have ever worked with [on Jimmy McGovern's Accused]. She said, 'I'm worried I will never work again', which I thought was hysterical." It is important to "be grateful and keep going". She adds: "Nothing is more diminishing than trying to control success or hold on to things."

How does she let go – relax? What would be an ideal holiday? "Right now, as a working mum: Four Seasons, Bali! No, actually, my favourite would always be a muddy tent holiday. Don't get me wrong: there is nothing more delightful than a dirty martini by the pool but I like being in nature." She would pick New Zealand because "there are no natural predators there". She goes off into another of her peals of laughter.

She believes she has changed now she is in her 40s: "I didn't really inhabit myself until I was in my 30s. And motherhood is an epic event. You can't help but be altered by it – and it is important to be." She understands better with age that "just because you feel something, that feeling isn't always the priority. I guess that is being an adult, isn't it?" The ways she lives have changed, too. "Pre-baby, I was a real yoga bunny." Can she still bend in every direction? "I'm pretty loose…" More laughter. But she doesn't officially exercise now. No time – acting is sport enough, alongside running after Brendan. "But I'm always dancing in my kitchen. And I love to sing. I've always sung. My father was a lovely singer. Always sang Jim Reeves at parties. I sing to my boy and he sings too."

Spark – spirit – is the key to Duff. She illuminates even the darkest roles. (She giggles about being cast as demented women like Edith Duchemin in BBC2's Parade's End – and hushes me before we can joke about whether this is typecasting. "She was bonkers, wasn't she? Like a mad racehorse.") As to spirit, she comments: "I am a sanguine individual. Most people are having a difficult time at the moment but still get up in the morning… As a species, we thrive. And I am interested in that ability to thrive. Things have to be about hope. On stage you need to convey hope or you'll lose an audience.

It is too soon to say how this will apply to Nina, but already the role is taking possession of her "as if drawing the calcium out of my bones". It is what director Howard Davies described as Duff's way of "throwing herself on parts as if bruising herself on them". She suggests she is "still that 19-year-old masochist that goes, 'F-ing great.'" And meanwhile, offstage, how much acting is involved just in being herself? "You know what? I can only be who I am."

Strange Interlude is at the Lyttelton, London SE1 from 28 May. The final episode of Shameless is on C4 on 28 May