Nymphomaniac star Stacy Martin talks sex, nudity and porn doubles

The unknown actress Stacy Martin, 22, is Danish film director Lars von Trier's latest muse. The former model with Premier Model Management, who was too short for the runway and looks like Jane Birkin, stars in his new movie, Nymphomaniac, which follows the erotic life of a woman from birth to the age of 50.

The provocative film in two parts, which features real sex scenes with porn doubles in softcore and hardcore versions, will premiere at the Cannes Film Festival in May.

Starring alongside Shia LaBeouf as his girlfriend, Martin is the younger version of Joe aged 15 to 31, a nymphomaniac, played later in life by Charlotte Gainsbourg – so its handy Martin looks like Gainsbourg's mother Birkin.

"What scared me wasn't the nakedness but it was interacting naked. Shia was very professional but we didn't actually do anything sexual," says Martin meekly, who is sitting in a café in London's Covent Garden. "That was left to the porn doubles. It's a totally different industry. The porn stars finish the job off for you. I never thought I'd meet my porn double," she laughs.

Martin remembers when she met von Trier in Copenhagen last year. She had always admired his films including Dogville starring Nicole Kidman, Melancholia starring Kirsten Dunst and Antichrist, starring Willem Dafoe and Gainsbourg.

"I'd only had three hours sleep because I was so nervous. I decided to stay awake on the plane so my eyes weren't puffy. Suddenly he was there in front of me, I was speechless. We just stood there starring at each other for a very long time," she says.

Von Trier and Martin talked a lot about what she could bring to the table as far as the character was concerned. "We were clear this film wasn't about nakedness, but that it was part of the story. So I was comfortable with it. Lars wanted to celebrate the female desire and show we have flaws, we are human beings and not perfect," she says. "There was a lot of pressure on me as the vehicle of his vision. He said to me, 'Just take it day by day, scene by scene. It will all come together.' There is an element of trust. I had to keep remembering that he chose me, or else I wouldn't be there if I wasn't capable. Sometimes I was rubbish and he allowed me to get that out of the way."

But it was challenging for Martin, who had no acting experience, to be thrust in front of the camera as LaBeouf's assertive girlfriend, with more naked scenes than most actresses have in a lifetime. It was filmed over 55 days in and around Cologne. The first part of the film is about Joe's younger life, the second part is when Gainsbourg takes over. The two parts of the film do not follow on from one another but are interwined.

"As the younger Joe, she is aware she is a sex addict but not to the point that it is a disease. At 15, she's just discovering men and friendships in the 1960s," says Martin. "It is a very interesting relationship she has with her boyfriend.

Martin, who is "itching" for her next role, joined an A-list cast including Uma Thurman, who plays one of her lover's wives, Christian Slater who plays her father, and Swedish actor Stellan Skarsgard, as an older bachelor, who listens to the nymphomaniac's life story told through Gainsbourg.

"In one scene I wear disgusting tight red high-waisted spandex pants which were absolutely vile with a zip and slutty tights and a tiny top like a handkerchief for a night out on the pull," she says in horror. "I looked in the mirror and kept telling myself, 'You can do this'. But the film is about a lot more than sex. There is a lot of humour which I hope people will understand. It is a story – it's not a porn movie."

The half-French Martin, who goes out with Daniel Blumberg, the singer of indie band Yuck, grew up in Paris. She lived in Japan from the age of seven to 13 with her dad, Rene, a hair stylist and her mum Annette – then she returned to Paris until she moved to London aged 18 hoping to make it as an actress. Modelling was just a side job to fund her life in London – she saved up to do private acting classes, studied the Meisner technique of acting at the Actors' Temple, and studied Media and Cultural Studies at the London College of Communications.

She had two auditions in London and one audition in Copenhagen for von Trier's film. "When a few weeks later I heard I got the part, it was like, 'OK, don't freak out'."

She says von Trier is "extremely quiet" on set and "gives a lot of freedom". "He doesn't give you tons of instructions, just lets you improvise. He will sit and see what you come up with. To work in that way is fabulous." A lot of the film was shot on a handheld camera. "Whatever happens, he gives a lot of power to the actors he works with. You are not like a puppet. He moulds the work rather than makes it concrete."

She says she never sat down with her older self, played by Gainsbourg to discuss the character but the two performances were allowed to merge naturally. "I haven't seen Charlotte's scenes but I've heard we match pretty well. Originally Lars didn't want to cast me so the fact I look like her mother Jane Birkin was not the reason why I got the part. I got it because I did a good screen test," she says.

Peter Aalbaek Jensen, von Trier's producing partner, claimed the director "wanted to see the sexual arousement of a girl" on screen. Skarsgard told The Hollywood Reporter the film is "sexually explicit but, believe me, it will be a very bad wanking movie". LaBeouf said: "I know he's a very dangerous director; I know we are trying to do something different."

The film goes back and forth over the years looking at Joe's life and her background – often scenes were filmed spontaneously. Martin says: "What was challenging to play was her determination. I'm the kind of person who will make sure everybody is OK. She is the opposite – she is very focused on what she wants and very mathematical. It was also challenging because it was my first film and it was with Lars and other big actors and I thought, 'This is a big shoe to step into'. But I just had to jump in there."

She is already excited about attending the premiere in a few months, but also wary. "I'm quite nervous about my parents seeing the film. My mum is naturally feeling quite protective. It's quite embarrassing for me to see myself have sex. I might be hiding my eyes."