The 35-year-old actor appeared on Jimmy Kimmel Live! on Wednesday, where he talked about working with Johnson for the duration of the entire Fifty Shades of Grey trilogy. Though the two have a steamy relationship on-screen, Dornan notes that off-screen, their bond is more familial.
"This is going to sound bad, but it's almost like brother, sister," he says with a laugh. "Because I am married, and she's had a lot of relationships in the time that we've known each other. You just have this sort of mutual love and respect for each other, and I feel like we know each other so well, and intimately."
As for their racy sex scenes, Dornan says the crew strives to give the actors as much privacy as they can while filming.
"They try to keep it a closed set ... only whoever really needs to be there, like me," he jokes.
When it comes to protecting his modesty, Dornan hilariously opens up about wearing a "wee-bag" to cover up his private parts.
"That's an expression I say from where I'm from, but it doesn't mean there's actually wee inside," he says. "I wear, like, quite a big bag ... like a huge travel case and I sort of shove everything in there."
Surprisingly, he later reveals the "wee-bag" has actually been used by another actor before!
"I'd love to get to the stage where you have your own one that you get to carry around," he says. "I think I was right to think this way: On the first movie, there was a selection of them, and I picked one. I thought you'd take them out of the packets and they'd be brand new; they'd have that brand new smell. Then I sort of picked one that I liked the look of and the shape of, and in the seam it said, 'Inmate No. 3.' It was sewn into it. I was like, 'Sh**. This has been used before?' When you picture the guy playing Inmate No. 3 in a movie in your head...I don't know. It's not sexy! ... Inmate No. 1, maybe."