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."