Author E.L. James is ''terrified'' by the pressure of making a movie adaptation of her bestselling novel, Fifty Shades of Grey.

The 50-year-old writer admits she feels huge pressure for Sam Taylor-Johnson's upcoming movie of her widely talked about book.

Speaking to Entertainment Weekly, James said: ''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.''

Jamie Dornan has been cast as Christian Grey after Charlie Hunnam dropped out of the role, while Dakota Johnson will play his love interest, Anastasia Steele.

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