Keira Knightley and her co-star Michael Fassbender revealed a little-known secret after filming an intense spanking scene together.

The pair starred in a David Cronenberg-led provocative drama about the early development of psychoanalysis back in 2011.

It centres around the characters of Carl Jung (Fassbender), Sigmund Freud (Viggo Mortensen), and Sabina Spielrein (Knightley), a patient suffering from hysteria.

Set in the early 1900s, Jung successfully treats Spielrein, a young woman, using Freud’s pioneering methods.

But as her mental health improves, Jung and Spielrein develop a romantic relationship, which strains his mentorship with Freud, leading to a massive rift between the pair.

Titled A Dangerous Method, you can take a look at the trailer below:  A Dangerous Method Official trailer

Now, during the film when Knightley’s character speaks about her traumatic childhood, Sabina engages in an intense affair with Jung, along with a bit of a spanking sesh.

Appearing on a recent episode of The Graham Norton Show, Knightley candidly admitted: “Basically it was about spanking. Michael had to spank me.”

To which Fassbender, 47, joked: “I was the spanker. You were the spankee.”

He explained: “To get the right level of spank, it can’t hurt too much, I suppose.

“There was a Yellow Pages between your bum and the paddle!”

The Pirates of the Caribbean star, 29, laughed: “I bought Michael and three other cast members their very own spanking paddles.

“I had to go into a sex shop and say, ‘Hello, can I have three spanking paddles, please?'”

Well, in an interview with Hollywood Reporter in 2011, Knightley said that she downed a load of shots before filming that scene.

“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,” she said.

“It’s gonna be everywhere; I don’t want that out there…I phoned him [Cronenberg] up to 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.”

Knightley starred opposite Michael Fassbender in the 2011 historical drama (Universal Pictures)

However, the director explained to Knightley that the scenes were going to be ‘clinical’ rather than ‘sexy’ or ‘voyeuristic in that way’.

“And I thought, ‘Okay, well I can understand that’,” Knightley continued. “As long as it’s clinical and it’s not some sort of weird, sexy spanking thing.”

She explained: “There was a box which he hit, so he was nowhere near me, thank God!

“I did actually say to Michael before one of the scenes…I was like, ‘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.’… [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!”