The 26-year-old actress was worried about the racy sequence from the first time she read the script, and admitted she needed to have a stiff drink before filming started and admits she is not keen to repeat them.
Speaking at the Toronto Film Festival, she said: "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."
The film - made by controversial director David Cronenburg - is an examination of the relationship between psychoanalysts Carl Jung and Sigmund Freud - played by Michael Fassbender and Viggo Mortensen respectively - and Keira plays Sabina Spielrein, a Russian who has an affair with Carl.
Keira was so nervous about the erotic scene, she even threatened Michael beforehand.
She revealed: "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.`"
For the actual scenes, Michael ended up spanking a box placed next to her. Keira was initially so wary of her spanking scenes she wasn`t sure she could take the role.
She told The Hollywood Reporter: "When I first read the script, I thought, `The script is 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.` "
She was, however, reassured when he told her the scenes wouldn`t be "voyeuristic" or too sexy.
She added: "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.`
"And I thought, `OK, well I can understand that. As long as it`s clinical and it`s not some, sort of, weird sexy spanking thing.` "