Johnson, who portrays Anastasia Steele, told Marie Claire US that filming those scenes were "never easy".
"It's not casual and it's not fun," she explained. "It never got easy."
One of the not-so-fun elements of filming the sex scenes included a sticky g-string.
"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,' Johnson, 28, explained
"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 f*cking bizarre."