Keira Knightley talks spanking in A Dangerous Method

How do you prepare to film a scene where you're going to be spanked by Michael Fassbender? If you're Keira Knightley in A Dangerous Method there's only one answer: vodka.

“I did a couple of shots of vodka - definitely - beforehand, and then a couple of glasses of champagne as a celebration of never having to do that again!" she told the Hollywood Reporter .

Of course, Fassbender – who plays Carl Jung in the story of his relationship with Sigmund Freud (Viggo Mortensen) – wasn't really hitting her, but that didn't stop Knightley issuing a warning.

“I did actually say to Michael before one of the scenes, 'I've got a security guard outside. You touch me and he's gonna break your legs!' And he was like, 'Keira, you're tied to a bed. You're not really in a position to say that.' I said, 'I guess you're right.'” she remembered.

The 26-year-old also revealed that she originally turned down David Cronenberg's film because of those scenes, but the director talked her into it.

"When I first read the script, I thought, 'The script's fascinating, and it's David, and I really want to work with David, but I read those two scenes and just went, 'I don't think that I can do that,' particularly because it's the age of the internet; it's gonna be everywhere; I don't want that out there.

“I phoned him up to really turn it down. I said to him, 'Look, I love you, I love the script, I love the character, but I really don't know that I can play those scenes'... He said, 'Look, if I'm gonna do them, then they're gonna be clinical; they're not gonna be sexy, they're not gonna be voyeuristic in that way,'” she said.