Twilight fans hoping to see Robert Pattinson and Kristen Stewart play lovers in the film adaptation of 'Fifty Shades of Grey are going to be gravely disappointed. The fast-selling erotic trilogy (Fifty Shades Darker and Fifty Shades Freed are the book's sequels) has caused a casting debate with both stars at the forefront. Shockingly, the author is against casting the Twilight couple, who have been quietly dating for more than three years.  

E.L James, who initially penned the books as Twilight fan fiction, doesn't think Robsten should star in the film version because it would be too strange. We're guessing she's one of the few people that hasn't seen their steamy Cannes Film Festival video.

The shy Brit told the outlet that she has about four actresses in mind for the role of the book's heroine, Anastasia and three potential candidates that could portray gorgeous billionaire Christian. James is unable to reveal particulars due to legal reasons. 

Stewart herself has admitted she's reading the book and even read a page aloud during an MTV interview. However, she has yet to express an interest in starring in the film. As for Pattinson, fans of the actor have created the Facebook page Robert Pattinson As Christian Grey In The 'Fifty Shades of Grey' Movie. Both actors are often included in the countless Fifty Shades casting polls that have hit the Web.

What James did open up about was the development of the two lead characters, Anastasia Steele and Christian Grey. The characters grew; they started that way and developed into completely their own people. It was fascinating getting to know them and know them very well, James told Red Carpet TV. I didn't see an outline straight away; I had no idea where these books were going to go. I just went with them. As I got to know them, they grew a life of their own. 

Since Fifty Shades was released last year, the book and its subsequent follow-ups have become a phenomenon. Currently poised atop the best-seller list, its raunchy content has led to an increase in sex toy purchases and hardware store items related to bondage. Adult companies, such as Pure Romance have even released a line of erotic products expressly for fans of the trilogy. Hotels mentioned in the books, such as the Heathman, are currently offering Fifty Shades packages.

Following a Hollywood bidding war, Focus Features scored the film rights for a whopping $5 million. A major reason James chose the studio over others such as Sony and New Regency that were also in the running was that it offered her casting approval.