Lena Headey is far too young to start lamenting the demise of the British film industry. She’s 29, and there isn’t a director in the country who would shout “next please” without a spasm of anxiety. Some have. Richard Curtis thought she was “too intense” for Love Actually, and put his chips on Keira Knightley. Others complained that Headey was too pretty to cast. Yet the actress curled up on the hotel sofa beside me in a pink shift, bare feet and jeans has more tattoos than a Somerset biker. Her grungy hair looks as if it’s been dyed to death. And her latest movie, Ol Parker’s Imagine Me & You, is a classic romcom with a frothy lesbian twist.

Intense? Dear Richard, I think the word you’re looking for is intelligent. Pretty? Of course she is. Lina Headey has none of the shiny,manicured celebrity of Catherine Zeta-Jones. She hasn’t banked a juicy Hollywood cheque. Yet she’s one of the most exciting and instinctive British actresses to emerge since Katrin Cartlidge. She can also do sex and siren parts, whether she’s trussed up in Russian corsets in Onegin or slumming it in a horror such as The Cave.

The tall and slender actress treats her screen Romeos much as she does her breakfast. She slides off the sofa, snakes across the carpet on her hands and knees, undresses a large bacon sandwich, and teases it mercilessly for 45 minutes. Her erotic performance as Angelika in Terry Gilliam’s The Brothers Grimm had much the same effect on Matt Damon and Heath Ledger. They responded like stage-struck hams impaled on the end of a fork. It is Headey’s most high-profile role to date, but it gave her little satisfaction.

“When I read for The Brothers Grimm I thought, this script is wicked fun, she says in her Yorkshire accent. “But the film was such a bummer. It leaves you empty despite the visual fireworks.” Her suspicion is that the film fell foul of studio politics. “The industry is under increasing financial pressure to play it safe for the American market. The excitement of making raw movies like we did ten years ago has all but gone.”

That said, she is genuinely excited about Parker’s feature debut. Headey is not, she insists, a lesbian. But her bitter-sweet performance is deliciously understated and surprisingly affecting. She admits that Imagine Me & You would have benefited if it had milked more lesbian sex from the relationship between the flower-shop owner she plays and the young bride she falls for at a society wedding.

Her old friend Piper Perabo plays the new wife who is torn between a baffled husband (Matthew Goode) and Headey’s bohemian charms. Their first taboo tumble in the roses should have been like Oliver Reed grappling with Alan Bates, I unhelpfully suggest. “I agree,” says Headey. “It was never going to be a big-issue film about being gay, but I remember hoping, ‘are we really going to get it on?’ The film was an utter joy to make. But personally I think it should have been hotter.”

For someone who has never had a formal acting lesson in her life, Headey has a fearless appetite to live the roles she plays. The emotional realism she brings to characters is the strongest part of her armoury. “I left school at 17 to act in films like James Ivory’s The Remains of the Day,” she says. “One minute I was in an NYT play. The next I was on a film set. I didn’t train. For every film I’ve always relied on an ability to access real feelings. I don’t know any other way. That’s probably why some directors think I’m too intense.” So did she feel obliged to fall in love with Piper on the set of Imagine Me & You?

“Yes. I think,” muses Headey, a tad hesitant. “Kissing her was very weird. We didn’t even have the luxury of being drunk. It’s easy to scream and shout, but to actually show intimate love in front of a camera is very hard. And almost impossible when you don’t feel something for the other actor. But the chemistry was nice between Piper and me. We’re old mates. We spent months together in wetsuits in Bucharest making The Cave.”

Headey is as disconcertingly candid about her appeal as she is about her films. When I ask if she’s ever worried about being cast for her “look” rather than her acting, she cocks an eyebrow and says: “Who knows, my friend? Maybe someone sees some talent in me. I’m not saying I’m a ‘minger’. But I seriously doubt directors cast me on looks and credits alone.” Her best work, she believes, is inextricably linked with directors who’ve been able to exploit her emotional gifts.

“Working with Stellan Skarsgard on Aberdeen (2000), a picture that had a tiny release, is still one of my favourite experiences.”

Her next film, Zack Snyder’s 300, inspired by the 300 Spartans who died fighting the Persians at Thermopylae, is apparently not short on blood or guts. Headey plays the Spartan Queen Gorgo and she relished the sheer brutality of the action and the extreme demands of the part. “Zack was offered a small fortune to bring the certificate down, but he said absolutely not. We’re all pretty much naked, and the battle scenes are full-on. It’s a fascinating film.”

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