Olivia Poulet: From Tory tough nut to a taste for the wild side

When I delicately suggest to Olivia Poulet, the actress who is best known for playing Emma Messinger in the hit BBC Four series The Thick of It, that she is typecast as a Tory ball-breaker, she balks for a few moments and looks slightly peeved, before saying: "Well, I'm definitely not a Tory! But I've had a lot of fun playing one."

The pretty, 33-year-old, who also played Carol Thatcher in Margaret, last year's acclaimed television biopic about Margaret Thatcher, and was Camilla Parker Bowles in the 2005 drama Charles & Camilla: Whatever Love Means, admits that television actors can be victims of "unadventurous" casting. In The Thick of It, a mock-documentary-style political comedy, she plays the goodie-two-shoes of the Conservative opposition; smart, toffee-nosed and full of attack.

Off-screen she has been famous for being the long-term girlfriend of Sherlock star Benedict Cumberbatch ("the thinking woman's crumpet"), from whom she split this time last year. In the flesh Poulet is enquiring, funny and much less assured than Emma Messinger. She tells me rather nervously about her upcoming role in Philip Ridley's new play, Shivered, directed by Russell Bolam, which opens at the Southwark Playhouse tonight. In a part which could not have shattered the mould more thoroughly, Poulet plays Lyn, a strange, other-worldly, woman who experiments with sexual violence to express grief after her son is decapitated.

Shivered is the latest in rash of Ridleys. The new effort will compliment a hugely successful revival of his 1991 breakthrough play The Pitchfork Disney, currently at the Arcola, and whet appetites for Tender Napalm, which will return to the Southwark Playhouse in mid April.

Ridley's work is typically dark and disturbing, playing on primal fears, taking inspiration from fairy tales and often featuring sexual outsiders. Shivered is set in a failed utopia, a new town built up around a car-production plant that has led to birth defects in the children living there. It is a tale told by seven people, across three families, and over 12 years, intertwining the horror of two horrendous acts (a decapitation and a bludgeoning) with surprising humour.

"A new play is always, obviously something of a changing beast," Poulet explains when we meet in a bar near to where she is rehearsing. "Philip has come into rehearsal to address things that haven't worked, or needed shifting around. But, on the whole, because his writing is so precise and he has considered every word so carefully, things have mostly been right as they were written."

Poulet admits that playing Lyn has been something of a challenge. The audience sees the character's war-hero son, Alec, returning from the front damaged and delusional. We then discover that, after his return to an unspecified war which could easily be in Iraq or Afghanistan, the 20-year-old is tortured and beheaded, with footage of his murder ending up on YouTube.

"Philip describes Lyn as like a unicorn in a muddy field full of donkeys. She's intelligent. High status. She was management level but worked in a canteen after she became a mum. She could have been, or done, something else. Which is why it is even more horrendous when the life she knows is completely smashed apart."

The collapse of her marriage, a difficult relationship with her disfigured son, and the arrival of a circus in town, culminate in a disturbing sexual encounter with a stranger at the derelict car factory where she, her husband and most of the town used to work. After reading the play, I struggled to see how this would follow, but Poulet eloquently explains:

"I've talked about it a lot with Philip, obviously. Lyn's son was tortured and beheaded, so her response [to seek out sexual violence] is partly the insanity of grief – she is trying to feel something. But also, part of her son's shell shock, or trauma, is that he started sexualising everything and, before he died, he accused her of being a slut."

In the script the character also accuses his father or having sex with men, and neighbours of prostituting young girls, and the character swings between normal and overtly sexualised behaviour.

Poulet's role is in huge contrast to her last major theatrical success, in Caryl Churchill's Top Girls, at the Minerva Theatre, Chichester last summer. It was the second time she had been directed by Max Stafford-Clark, and the production received universally rave reviews.

"During Top Girls, when I left nightly with that intense feeling everybody was having a riot, I told myself that from now on I'd do nothing but comedy. And here I am doing a Philip Ridley! That's not to say there isn't comedy in it," she grins. "But the audience isn't going to be skipping out of the auditorium." Poulet's funny bone is still being exercised in collaboration with actress Montserrat Lombard, with whom she is writing a feature film ("it's an odd, indie road-movie thing").

Poulet has written several plays, including The Bird Flu Diaries and The Queef of Terence, both of which she took to the Edinburgh Fringe. She has recently finished a screenplay based on the true story of four girls who rowed across the Atlantic. "I love writing. I get excited by it and it's rewarding in a way that is totally different from acting. The dream is to write my own stuff. I think that, as a woman with a comedy slant, it is a great thing to be able to generate your own work."

Despite her frankness about the torment of being a struggling actress, the bursts of success followed by agonising stagnation, she doesn't mention Cumberbatch and the inevitable comparisons that a couple in the same businesses will draw. They were together for 12 years, having met at Manchester University, and they separated last March when Cumberbatch was being praised to the rafters for his performance in Danny Boyle's production of Frankenstein. I venture a question about Cumberbatch. She bats it away with: "I don't want to talk about that." Visibly rattled, she quickly says: "Everything's fine. We're fine. Still really good friends."

Poulet starts filming the next series of The Thick of It in late April and is excited about what's in store for Emma. "At the moment we're all sending in jokey ideas to the writers." Emma Messinger for PM? "Why not? She could take them on."

'Shivered', 13 March to 14 April, Southwark Playhouse (020-7407 0234)