E.L. James, the author of the saucy Fifty Shades trilogy, has revealed that she wrote the books as part of her midlife crisis.

It's all my fantasies in, in one thing, she said in an interview with ABC's 20/20 that will air on Friday.

James' story is about a college student and the rich, handsome man with whom she engages in a steamy affair. It started off as Twilight fan fiction -- originally titled Master of the Universe -- with Bella Swan and Edward Cullen in the leading roles.

Vintage Books, which acquired the U.S. publishing rights to the books, defended the author against any accusations that her story is a rip-off of the Twilight books, which were penned by author Stephenie Meyer.

It is widely known that E.L James began to capture a following as a writer shortly after she posted her second fan fiction story, Vintage said in a statement, according to an Associated Press report. She subsequently took that story and re-wrote the work, with new characters and situations. That was the beginning of the 'Fifty Shades' trilogy. 

James described her story to ABC as a contemporary, romantic fiction, with quite a lot of sex in it and said that her teenage sons have not read the books.

I'd be mortified, and they'd be mortified, you know, she told ABC. It'd be far too embarrassing.

The story's popularity has sparked many a debate, particulary about the sexual nature of the relationship between the two main characters.

Add to that a potential movie looming on the horizon and you officially have a frenzy. Universal purchased the movie rights last month and is partnering with Focus Features, which will do the filming and distribution, according to the Hollywood Reporter.

Fans are eagerly speculating about who should be cast in the lead roles of Anastasia Steele and Christian Grey. Actresses Lily Collins, Lucy Hale and Shailene Woodley are among the names being suggested. Actors that have been touted for the role of Christian Grey include Henry Cavill, Ryan Gosling and Ian Somerhalder.

James' interview with ABC's 20/20 will air on Friday at 10 p.m. EST.