Sam Taylor-Johnson Covers Red, Reveals "Hardest Thing" About Directing Fifty Shades of Grey

Plus, find out why she says star Jamie Dornan is "funny" and "a sweet and lovely man"

Sam Taylor-Johnson knew directing Fifty Shades of Grey, the movie, would be quite an undertaking.

She understood, too, that the film, based on the first book of E.L. James' erotic trilogy, had a very enthusiastic, built-in fan base, so rest assured: She made every effort to stay true to the book on the big screen! "I've practically eaten it," Taylor-Johnson, 47, tells Red magazine's March issue. "I have read it over and over."

She admits she had to set some hard limits when it came to keeping creative control. "I want to protect my vision and that's the hardest thing," she says. "There are so many voices. You have to try and keep all those people at bay. You think, you hired me because I'm a creative artist with a vision. Don't try and knock it out of me."

And from start to finish, directing Fifty Shades of Grey has been stressful. "I'm literally going out of my mind," she tells Red, speaking to the mag during the film's editing process. "I have seen this film over a thousand times and I'm now in a blancmange. But the nuances within-the tiniest shift, look, blink of an eye-can make all the difference and flip it into the wrong territory. It's like walking a knife edge."

Fortunately, Taylor-Johnson found Jamie Dornan, the actor playing Christian Grey, to be "a sweet and lovely man."

"Jamie's funny because he doesn't seem to feel any of the pressure," she says. "Or show he feels it. Everything washes over him and he's constantly happy."

This laid-back, good-natured attitude undoubtedly served 32-year-old Dornan well throughout filming, even as he and Taylor-Johnson "had to sit and meet various dominants and dominatrix, just to make sure we understood that world; so that we in no way portrayed it incorrectly," she says.

Learning about BDSM with your boss would have to be awkward! For Taylor-Johnson's full interview, pick up the March issue of Red, on sale Jan. 29.

Fifty Shades of Grey hits theaters Feb. 13, 2015—just in time for Valentine's Day!

(E!, Universal Pictures and Focus Features are all part of the NBCUniversal family.)