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In the David Cronenberg film, Knightley plays Sabina Spielrein, a Russian patient, who is treated by Sigmund Freud and Carl Jung and overcame her mental illness to become a therapist and colleague of theirs.
The British actress, 26, had to film several sex scenes with Irish-German actor Fassbender, who plays Jung.
One risque scene sees him spanking Spielrein, but speaking at the UK premiere of the film at the 55th London Film Festival, Knightley said it was all just pretend.
Actor Michael Fassbender plays Knightley’s lover Carl Jung in the film
“I actually said to him before we shot the spanking scenes ‘If you touch me I will kill you’,” she explained.
“He did remind me that I was tied to a bed at the time and really I shouldn’t be making any threats.”
Knightley said those scenes were not “particularly easy” but filming with Fassbender was “wonderful”.
She told Sky News: “I am so incredibly lucky to be part of this film, Viggo Mortensen, Michael Fassbender and David Cronenberg. It was an extraordinary, extraordinary experience.”
Knightley with co-stars Fassbender and Viggo Mortensen
Knightley’s real life character helped to create ideas that influenced both Freud and Jung.
She said: “It was an extraordinary opportunity to play somebody so inspiring.”
The Pirates of the Carribbean star shot to fame after starring in football film Bend It Like Beckham in 2002 which is now being turned into a musical.
Knightley said it sounded like an “exciting” project but she will not be taking to the stage herself.
“I did it when I was 16,” she told Sky News, “and it was wonderful and I loved everybody but, no, I think I will leave that as a 16 year old.”
The actress commented that there is an ongoing Russian theme running throughout her current work. She is busy filming a version of the Tolstoy novel Anna Karenina.
But contrary to reports she said she has not been learning Russian, admitting, “I haven’t never even been to Russia.”
