On my way to meet Tamsin Greig I make a resolution: I will not mention The Archers - in which she plays Debbie Aldridge - until well into our interview; more especially I won't ask what on earth Brian Aldridge thinks he's doing splitting up the farm, and why isn't Debbie hotfooting it over from Hungary to give him a piece of her mind in that icy, implacable voice she uses when confronting her stepfather?

Because at 41, Greig is no longer to be defined as Debbie Aldridge, or “the hapless” Dr Caroline Todd in Green Wing or dotty Fran Katzenjammer in Black Books or even endearing Alice in Love Soup. No, Greig, who is appearing with Ralph Fiennes in the new Yasmina Reza play, God of Carnage, at the Gielgud Thetare, has rather suddenly become a stage actor of some note. She has played lead roles with the Royal Shakespeare Company, winning an Olivier award for her almost shockingly sexy portrayal of Beatrice in Much Ado About Nothing (décolletage and pelvic thrust).

Her small dressing room at the Gielgud contains a bed and two chairs: “Would you like to lie down?” she quips. I am about to say that it is she who should rest, what with rehearsal this afternoon followed by preview performance, when I catch sight of two pairs of identical shoes - black, very high heels, crisp pleated bow on the toes - and have a Nancy Mitford moment: “Fanny, the shoes!” Greig nods: “I know, they are divine, aren't they?” Can she walk in them? “Well I practised a lot - it's a bit like stilt-walking - but I wanted my character to have very high-heeled shoes. She is slightly unsure of herself and I wanted to feel a bit frightened when I played her. That's as close as I get to the Daniel Day-Lewis method.”

The play is about two couples who don't know each other but meet up to discuss the unruly behaviour of their respective sons, an evening that degenerates into tears, tantrums and physical fights. “They are middle-aged, middle-class people with problems they should be able to deal with in a mature way,” Greig says. “But as adults, sadly, we rarely let go of our innate selfishness and desire to be right.”

So it's a battle to the end? “Yes,” she nods, “It's very black comedy. In previews there has been this nervous, defensive laughter. It's like, ‘Oh God, I've so done that, said that'. There is recognition, relief and release but also terror.”

Her high heels make her only a little shorter than Fiennes, she says, and bring her closer to Janet McTeer, the play's other female character, who is famously tall as well as famously accomplished. “In the dressing room next door,” Greig whispers, cupping her hand round her mouth like a schoolgirl, “I can hardly believe it. And down there,” - she points through the floorboards - “Ralph Fiennes and Ken Stott.” The eyebrows shoot up: How did that happen?”

This is not, I think, disingenuous. She claims she looks like a puffin and has dismissed successful television roles with the words, “basically, my job is to look surprised on the telly”. She seems genuinely surprised at a career that began with rejections from every drama school in England (she did a drama degree at Birmingham in the end), then bobbed along modestly through the Nineties with The Archers and occasional TV roles. Then, in 2005, after the birth of her third child, she found herself simultaneously filming Love Soup and Green Wing, popping up to Birmingham “to do Debbie”, as she calls it, to Cardiff for Dr Who, and breastfeeding the new baby in dressing rooms and studios all over town.

How does she manage with three young children? (She has sons of 9 and 7 and a three-year-old daughter.) “I was very blessed,” she says. “When I first had children I had no huge career to surrender, no great dilemma about work versus family. If someone had said back then: ‘One day you will be in three TV shows and radio and breast-feeding a baby,' I'd have said ‘no way'. A career on a slow burn has been perfect for me.”

When the parts started flooding in, she says, the Archers team were very accommodating: “They sent Debbie off to farm in Hungary - a legitimate, very now thing for a young farmer to do - and made it possible for me to record my occasional appearances down the line.”

Unlike Greig, Debbie is pretty humourless; does she like her? “I understand her. Her father abandoned her, her stepfather betrayed her mother, her husband left her: you need to see the ridiculousness of a situation to be funny, and it's too painful for her.”

When Greig was offered parts in Much Ado and King John in the RSC's 2006/07 Complete Works season, she knew it would mean living away from home for seven months and giving up lucrative television work. “I thought, I can't leave the children; I can't support five people on that wage. Six, if you count the nanny.” Her husband, the actor Richard Leaf, was not so sure. “He's such a yes man,” she says. “He goes: ‘Let's think about this.' He sees a solution before he looks at the problem. I'm the other way; I go: ‘Oh no, Oh no! Look what's happening!'” She flaps her hands comically. In the end, they managed: the nanny went part-time; the grandparents stepped in to look after the children two days a week and Greig came home from Stratford whenever she could. “I felt very responsible,” she says. “Eight people's lives were affected. But now I think it was good for me to get out of the picture: you tend to think, as a mother, that you need to be in the centre otherwise everything will fall apart.”

And her Beatrice was a triumph - eventually, anyway, once she'd learnt how to make herself heard.[] “I'd had a ten-year gap from the stage, relying on TV and radio mikes. I'd lost the knack. My husband came to see a preview and said afterwards: ‘You were delightful but it would be good if we could hear you.'” She pulls one of her large repertoire of dismayed expressions. “Then Janet McTeer told me: ‘You will be heard when your character wants to be heard,' and I thought, yes! Here you, Beatrice, are, in pre-revolutionary Cuba; you're a woman and if you open your mouth you may be killed. If you want to be heard, you need to do it with balls.”

So now it's a three-month West End run for God of Carnage and then what? “You never know what's going to turn up,” she says. “You just keep versatile and lithe.” Does she ever feel guilty, being more successful than her husband? She frowns: “In my most self-obsessed moments I might. But he is so generous, celebrating the things that happen to me. He's doing much more writing now, and has things in development. He has this fantastic other talent as a writer that I don't have.” She looks mournful: “Trampolining - that's the only other thing I can do.” She brightens: “Anyway, a career doesn't really exist, only in people's minds. Other things - relationships, family, they're real.”

And faith, she might have added. She has been a committed Christian from the age of 30, though she was not brought up to believe: “My mother was a determined aetheist after she was rejected by the Catholic Church for marrying a divorced man.” Her father “never showed his emotions”, she says, but the two of them shared a love of slapstick humour. He died in 1998 telling her: “I'm glad you've found what you've found. I always prayed you know, every day. In secret.”

Her own faith crept up on her - a quiet insistent thing, she says: “A feeling of ‘is this it?'”. But she is reluctant to talk it up. “People try to challenge it. Somehow they find it easier to share grief than joy.”

What makes her happiest? She thinks for a moment: “The other day I was driving along, looked in the mirror and saw all three children laughing their heads off. Whatever it was about, it was nothing to do with me. I thought, I pray these three will laugh together all their lives.”

God of Carnage is previewing at the Gielgud Theatre, London W1, and opens on March 25 (0844 4825130)

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