Jaime Winstone’s jaw has dropped and her face has turned ashen pale. The 23-year-old actress and London party maven is talking about sex: movie sex, deviant sex, drunken sex, drug-fuelled sex and flat-out take-control-pin-him-to-the-chair-and-lose-yourself-and-find-yourself-sex, all of which have a pivotal place in her controversial new thriller, Donkey Punch.

She is sitting outside an Edinburgh hotel, just before the movie’s European premiere, and bantering with co-stars Nichola Burley and Sian Breckin. They are discussing the film’s orgy when a shocking thought hits Winstone for the first time. “My boyfriend’s going to see this tonight!” she gasps. “I’ve suddenly realised that, and I’ve got the fear!”

The “boyfriend”, of course, is Alfie Allen (brother of Lilly). The “fear” is his imminent reaction to the sight of Winstone naked and wrapped around her smoothie co-star Jay Taylor in a “youth orgy” sex scene that is both explicit and lengthy, and provides Donkey Punch with a crucial dramatic fulcrum.

For here, while portraying a group of English party animals moored on a yacht off Spain, and high on drugs, Winstone and co are introduced to the mythical titular sex act (a sharp blow to the back of the neck, mid-coitus, that is alleged to induce muscle spasms in the unlucky recipient and exhilaration all round), but with devastating consequences. “Just tell him, like, ‘I’m sorry, I was thinking of you!’ ” says 22-year-old Leeds native Burley, chuckling as she comforts Winstone (the two actresses have been bosom buddies since they bonded on the Channel 4 drama series Goldplated). Burley turns to me and adds: “I remember walking on set during the sex scene, and I didn’t know where to look. Everyone was naked. All you could do was stare into their eyes.”

“But it was sensitively handled,” says Breckin, 26, the screen novice of the trio, with only theatre credits. “Some days we were like, ‘Is this what it’s like to be on a porn set?’ But mostly it was about kids who are just having a great time, and who shouldn’t be judged for having sex.” Yes, but what about the donkey punch? It gives the movie a radical edge, and it provides a creepy mix of sex and violence that defines the hipster tone of the whole project, but does it even exist? Have you lot, for instance, ever tried it? “Absolutely not!” says Breckin. “It’s the kind of thing that men talk about but no woman would allow to happen.” “No it happens,” counters Winstone. “Of course it does.

There’s a lot of kinky people out there.” “But realistically,” adds Burley, “if you hit someone on the back of the neck, with force, during sex, you’re obviously going to break it!” “Let’s turn it around,” says Breckin with a grin. “We are women here, but would you do a donkey punch?” I tell them that I’m not a fan of the punching, but could work my way up to some heavy neck massage perhaps. “Awwh sweet, a donkey massage!” sneers Winstone. “A donkey rub!” giggles Burley. “Darling! Time for your donkey rub!”

The actresses first encountered the script for Donkey Punch when it was whizzing around British agencies like the proverbial hot potato. “I remember thinking, ‘Wow, if this ever gets made, it’s going to be really f***ing powerful!’ ” Winstone says. Co-written by David Bloom and the director Oliver Blackburn, the film was inspired by a stag night Bloom attended, where the men were bragging about sexual antics. “When the donkey punch was mentioned it silenced the room,” Bloom says.

The finished screenplay is remarkably high concept. Four London boys pick up three northern girls at a Spanish resort, bring them out to sea on a yacht, and perform a fatal donkey punch. Thereafter all aboard begin to murder one another. It could have been completely crass, like Hostel on a boat. Yet the surprise hook in Donkey Punch is the depth of characterisation, and the seemingly naturalistic bonds that exist between the protagonists.

“That’s because we were all really close during the shoot,” Winstone says, explaining how the film-makers forced the girls to flatshare for the entire six weeks of the Cape Town-based shoot (though set in Spain, South Africa made more fiscal sense on the film’s modest £500,000 budget).

The proof of the making, however, is in the screening and Donkey Punch wowed viewers at this year’s Sundance Film Festival. “The audience at Sundance were standing up, cheering and shouting at the screen,” Burley says.

And yet what about the inevitable controversy when the movie reaches mainstream audiences? What about the popularisation of a dangerous sexual practice? “We should have a warning at the end saying, ‘Don’t try this at home kids!’ ” Burley says. “Unless you have an adult present!” adds Winstone, cackling loudly and sending her two friends into uncontrollable fits of healthy, filthy, girly giggles.

Donkey Punch is released nationwide on Friday 18 2008

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