It's not the first time the 26-year-old English actress has shot a sex scene, and in 2008 she told reporters that she didn't mind appearing nude for the right role.
But while there will always be actors willing to disrobe for a film - either because they have no reservations about it, or feel compelled to do so to land a part - some, like Knightley, are becoming increasingly wary of onscreen nudity, knowing the images will be catalogued and archived online forever.
"It's not like doing it in the '60s or '70s - when I think you could do things and they were kept to the medium they should be kept to - now you've got people who want to take them out of any kind of context," Knightley said during a recent interview.
"If you watch the films at the moment I think there are less and less actresses who are taking their clothes off, who are doing scenes like that, and completely understandably because there is all of a sudden a whole other side of it that you do have to think of."
But even Knightley is prepared to make the odd exception, as have several other major stars with films at this year's Toronto International Film Festival.
From Michael Fassbender's full frontal nudity in "Shame," to a decidedly unsexy public shower scene in "Take This Waltz" and the symbolic nudity in "Melancholia," there's no shortage of festival flicks that feature prominent uncensored views of the human form.
In the case of "A Dangerous Method," about the birth of psychoanalysis spearheaded by Carl Jung, his mentor Sigmund Freud and the largely unknown pioneer Sabina Spielrein, Canadian director David Cronenberg envisioned a sadomasochism scene as crucial to the understanding of Spielrein's relationship with Jung.
Knightley relished the chance to work with Cronenberg and adored the script for "A Dangerous Method," but had "huge reservations" about sex scenes with co-star Fassbender.
It took a lot of talking with Cronenberg before she finally agreed to take on the role - and take off her clothes for it.
"I actually kind of thought about turning it down right at the beginning," Knightley recalled.
"And (Cronenberg) said, 'OK, well it'd be a real tragedy if you turned the part down so we'll take the scenes out, we don't have to shoot them.'
"And I went, 'No, no, because I think it's incredibly important that they were there.... And we talked about it and talked about it and once he said, 'I don't want this to be sexy, I don't want it to be voyeuristic, I want it to be clinical ... so you absolutely understand what her character is,' I said, 'OK, that makes sense to me and I can do that.'"
For his part, Cronenberg said he really was willing to compromise.
"I said to Keira, 'It's your comfort level ... we could do (the spanking scene) with total clothing on if we needed to,' and so we just had a very heart-to-heart talk on what she'd be comfortable with and what she wouldn't," he said.
"I say comfort, well, I'm sure she wasn't totally comfortable. But she was really willing to give that to the movie and so it was a very straightforward discussion. It was nothing like me forcing anything, it was a collaboration.
"I wanted Keira in the role, if she said, 'I will not do any nudity,' I would've still wanted her in the role."
Continued...