Keira Knightley may be a bona fide Hollywood star, but she still has strict standards when it comes to filming sex scenes.
The actress' latest movie, crime thriller A Dangerous Method, sees her partake in several kinky sequences with Irish co-star Michael Fassbender (pictured above right), including a spanking scene.
But appearing at the premiere in London last night, Knightley, 26, said she had issued a stringent "no touching" guideline for the entire process of filming.
"I actually said to him (Fassbender) before we shot the spanking scenes, 'If you touch me I will kill you' and he did remind me that I was tied to a bed at the time and really I shouldn't be making any threats," she said.
"The film is about so much more than S&M but (those scenes) weren't particularly easy," she went on.
The movie explores the intense relationship between Carl Jung (played by Fassbender) and Sigmund Freud, with Knightley in the role of psychoanalyst Sabina Spielrein.
Knightley, who wore a Roksanda Ilincic dress and Salvatore Ferragamo heels, said she welcomed the challenge of the part.
"It's important to push yourself," she said. "I don't want to do the same thing again and again and again and I don't think people really want to watch that, and sometimes I'm going to fall flat on my face and I think I have done that, and will continue to do that but hopefully there will be some gems in there that are really interesting as well."