"A lot of getting into the character's head, in Ana's headspace before she meets Christian, so it's a lot of reading, which I love," Johnson told Vanity Fair.
"She's an English major, so that's kinda boring," said Johnson, who features on the magazine's March issue.
The actress says shooting the film with Jamie Dornan and the rest of the cast and crew has been very positive.
"It's been a very smooth and fun and creative experience. I think going into something like this, you kind of have fears of what it could be like and none of those fears are happening. We all get along really well and the crew's amazing. We're just all trying to make this project the best it can be," she said.