The beauty plays a Russian woman with severe psychological issues who comes between famed psychiatrists Carl Jung and Sigmund Freud, in the film.
She admits that she had almost turned down the role because of a scene where her character, Sabine, is violently spanked.
So, she warned Fassbender she would take revenge if one of the fake slaps actually struck her, reports dailystar.co.uk.
?I actually said to him 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,? said Knightley.