E.L. James is ''terrified'' about the film adaptation of 'Fifty Shades of Grey'.

The author admits she feels huge pressure for Sam Taylor-Johnson's upcoming movie of her best-selling book to be a success and she is determined to get things ''right''.

James told Entertainment Weekly: ''I'm terrified. Completely. I've been terrified from the moment I published the book. My mother was terrified of everything, and so am I. It's a terrible way to grow up. You don't expect this kind of success.

''Even now it floors me. My only ambition for the books was to see them in bookstores. This is huge. And there is this passionate fandom; we need to get this right for them.''

Irish hunk Jamie Dornan has been cast as S&M-loving billionaire Christian Grey after Charlie Hunnam dropped out of the role, while Dakota Johnson will play his naïve love interest, Anastasia Steele.

Max Martini is on board to play Christian's bodyguard, while 'Pride and Prejudice' actress Jennifer Ehle will star as Anastasia's mother.

James initially had reservations about agreeing to the movie, but decided the opportunity was too big to pass up.

The author explained: ''I didn't know if it was the right thing to do. And then I thought, 'I'm middle-aged; when in the hell am I going to get another chance to make a movie in Hollywood?'

''That was the final thing: Let's go for it. Let's see if we can have some fun with it.''

'Fifty Shades of Grey' is scheduled for release in February 2015.