Shia LaBeouf didn't actually have sex with his co-star in 'Nymphomaniac'.

Actress Stacy Martin knows scenes between her and Shia are particularly intimate in the movie - about a self confessed sex addict recounting her life story - but they were achieved through careful use of prosthetics and porn doubles.

She told Nylon magazine: ''Neither Shia nor I were willing to actually have sex onscreen, obviously.

''Everything was set in place for us to have a porn double, a prosthetic vagina, special effects.

''As actors, we could be ourselves and commit to an amazing project, but we were protected as human beings, and, speaking for myself, as a woman.''

Stacy, 22, also said the script led to her and Shia to have some very interesting conversations off-set.

She added: ''We'd be sitting around in restaurants, saying, 'You know the penis scene? How are we going to do that?' ''

The budding actress has previously said filming some of the sex scenes got a little boring because they were so technical.

She said: ''It's not the most relaxing experience. It is quite strange, but also it is very, very technical. It almost becomes a bit boring because [director] Lars [Von Trier] gives you a lot of freedom when you're filming in general, and then suddenly you are restricted into doing certain positions and being very specific. Because they have to match everything together [with the doubles], you just have to really obey what they tell you. So you just have to get on with it.''