Jamie Dornan reveals the one sex toy that didn't always work during Fifty Shades Darker filming

Light, sexy spoiler warning!

It’s no secret that the filming of steamy movie sex scenes is more often than not the opposite of sexy.

Take Fifty Shades of Grey sequel Fifty Shades Darker for instance, where sex plays a central role in the plot (if you weren’t already aware of this fact, then you haven’t been paying attention to the film coverage or anything about the E.L. James-authored books which inspired them) — and where the specially designed sex toys for certain sex scenes don’t always “perform” in the manner in which they were designed, like the “spreader bar” with a special twist.

“The spreader bar, they made that,” star Jamie Dornan (aka Christian Grey) tells PEOPLE of a memorable device referenced in the books and put into convincing practice in the film. “They were actually really struggling to get their hands on that.”

Dornan says that the toy in question was born from author James’ imagination and doesn’t exist (yet), so the prop department on the film had to get creative and make it.

“It didn’t work a couple of times and held us back a little bit,” Dornan recalls with a laugh. “It didn’t quite achieve what we wanted it to achieve, but when it mattered it worked, so it was all good.”

For much more on Jamie Dornan and Fifty Shades Darker, pick up the latest issue of PEOPLE, on newsstands Friday

Dornan performs a particularly impressive athletic maneuver in the scene (no spoilers) which involves using the toy to flip his costar Dakota Johnson (aka Anastasia Steele) over. The edited version in the film is as sexy as it is fluid — even though Johnson was giggling and possibly snorting during a few takes.

“When [Jamie] did that and he flips her over, she giggles a little bit, which I love,” says Darker director James Foley of Johnson’s honest reactions captured on camera. “Because I felt like she’s having fun, it’s a playful thing, so the audience can have fun and not think, ‘Ooh is this weird?’ It’s all about play.”

RELATED VIDEO: Jamie Dornan Reveals How He Changed Christian Grey in Fifty Shades Darker

Sometimes during filming, however, Johnson got a little carried away with the “play” part and lost herself to fits of giggles and snorting.

“Dakota is a snorter, she does that all the time,” says Dornan. “You know you’ve really got her when she snorts, when she’s laughing so much that she starts snorting. And then her snorting makes her laugh even more, so then you’ve got to let her bloody [go on], just leave her to it. She laughs, then she’s crying, makeup has to come in and redo the mascara or whatever it is.”

Dornan adds that he and Johnson laughed together, and frequently.

“We had a lot of fun on this job and made each other laugh a lot,” he says. “But that’s what work should be like I think, especially the more tense the scene is, the more I think we should be laughing.”

Fifty Shades Darker opens on Friday.