Robert Pattinson said filming X-rated scene was ‘one of the most difficult things has ever had to do’ for one reason

Robert Pattinson dished the dirt on what it was like to film sex scenes for the final Twilight films, revealing the surprising reason why they were particularly difficult.

Landing the role of Edward Cullen in the film adaptation of Stephenie Meyer's vampire romance fantasy transformed Pattinson's career trajectory, taking him from an actor who was known for his supporting role in Harry Potter and the Goblet of Fire to a verified teen heartthrob.

The franchise has since been cemented as part of modern pop culture - thanks to viral phrases such as 'Hang on tight, spider monkey' and 'Bella! Where have you been, loca?' - meaning its easy to overlook some of the more cringeworthy moments.

Chief among those moments were Breaking Dawn's highly anticipated sex scenes between still human Bella (Kristen Stewart) and vampire Edward Cullen (Pattinson).

Pattinson's role as Edward Cullen made him a household name (JB Lacroix/FilmMagic)

But it turns out they may have been just as awkward for the actor to film as they were for us to watch.

Speaking about the filming process in an interview with Parade after the premiere of 2011's Twilight Saga: Breaking Dawn, Part 1, Pattinson explained why the intimate moments between him and Stewart were hard to capture on camera.

"I think the more comfortable you are with each other, the harder it gets to do any kind of really intense relationship scene," the Tenet actor explained.

"Watching other people have sex is never going to be that spectacular anyway," he continued, adding that there was a pressure to live up to the 'hype' from fans.

"You are like, 'God, I hope this lives up to it.' The toughest part was avoiding an R-rating.

"It’s funny when people talk about the sex scenes in the book because there are no sex scenes in the book. Go back and read. They always fade to black."

However the Breaking Dawn's intimate scenes weren't always the easiest to film (Summit Entertainment)

Pattinson also revealed that romance scenes shot in the ocean on Edward and Bella's honeymoon were particularly hard to get right, due to the practical difficulties involved in the location.

"It was impossible [to film] in the waves," he said.

"We were trying to kiss each other and we misjudged the strength of the tide and so it was absolutely one of the most difficult things I’ve ever had to do in my life."

Edward and Bella's wedding night isn't the only time in which Pattinson has acted out steamy scenes on camera, as the 39-year-old previously masturbated on camera for 2008's Little Ashes.

However they're not Pattinson's most x-rated moment on screen (Regent Releasing)

Explaining why he decided to actually rub one out rather than simulate the moment in an interview with German magazine Interview, Pattinson said he felt unable to fake it effectively.

"Try it. I can tell you right now, no chance. It just doesn’t work," he said. "So I rubbed one out in front of the camera... my orgasm face is recorded for eternity.”

Wonderful.