ET's Cameron Mathison caught up with the co-stars exclusively at Paris' Palais Garnier opera house on Monday, and the 28-year-old actress shared how the two got in the right mental space to act out the erotic scenes in the book-turned-movie.
Johnson -- who portrays leading lady Anastasia Steele -- revealed that it took some "serious psychological preparation" on her part. "I mean, there is a lot more of my body that's shown than Jamie's actually," she said, adding that "weirdly he's the one doing all the push-ups."
While this is their third movie in the film franchise, Johnson admitted that the red room scenes are "not something that's easy to do."
"I really had to get ready for it each time and know exactly what we were going to do and how we were going to accomplish shooting the scenes, the more intimate ones," she explained. "Over the course of the three films, it never got easier. It wasn't like, 'Oh, this old thing,' you know?"
Praising her bond with her co-star, Johnson added, "Luckily, Jamie and I were so comfortable with each other and it felt safe and protected, but it's still an environment that's scary."
When talking to ET, Dornan noted that he negotiated having more clothing on for some of the scenes when he thought it worked best for his character. "I tried to do a lot of negotiating on a lot of different things," Johnson said when asked if she too attempted such arrangements.
That being said, Johnson completely understands that the sex scenes are a big plot point in the films. "I know what I signed up for," she noted. "It's not like they are forcing me to take off my clothes. I choose to do these jobs."
Dornan also opened up to ET about what it will be like for him to say goodbye to the character of Christian Grey. "It's tough in some respects, but it's also really nice and kind of bittersweet, I guess," he said. "I think when you finish any movie or TV series or whatever, you've invested so much [time] into the character and the job itself and the other actors and the crew."
Fifty Shades Freed hits theaters on Friday, Feb. 9.