Dakota Johnson and Jamie Dornan worked with sex toy experts on the Fifty Shades films, so they looked like they would know what they were doing during steamy scenes.

The couple's characters, Anastasia Steele and Christian Grey, test the bounds of sado-masochism in the movie trilogy, based on E.L. James' erotic books, and to make sure they looked like old pros, there were consultants on set to show them how to handle whips and other sex den necessities.

"We just had people who specialised in knowing how things work, like how to use certain tools and toys," Dakota tells MarieClaire.com. "There's a certain method. It's very intricate and the details are really important. The rules are also really important. We didn't want to go make a movie about something we didn't research to the bone. So to really honour the BDSM community and we did it the right way."

But that didn't make the sex scenes comfortable - and one in particular left Dakota battered and sore.

"The most difficult one (scene) by far, I think in all three movies, was a sex scene in the third film, where I was handcuffed with my arms and legs to the gate," she recalls. "It's like a fence thing that comes down. And I was blindfolded. And it was kind of shocking, because I didn't realise that no matter how prepared or rehearsed I was, when your senses are taken away like that, you can't control the way your nervous system will react.

"It was really tricky to navigate that scene. It was kind of difficult."

And she still can't believe the silly things she had to wear for the steamy scenes in which she appears to be naked: "I had these sort of strapless thongs that had glue on them. It's not glue, but it's sticky," she giggles. "They're, like, basically if it were a nipple pastie, but underwear. But it's only sticky at the top, it's not sticky the whole way.

"It would also come off because the adhesive would wear out, so then they would superglue it to my body so that it wouldn't fall off. And I would wear two of them. It's not painful... (but) it's f**king bizarre."