A series of famous faces have been linked to the lead roles of Christian and Ana, including stars such as Ryan Gosling, Shailene Woodley, Angelina Jolie, Emma Watson, Katie Cassidy, Channing Tatum, Lucy Hale, Alexander Skarsgård and Ian Somerhalder.
However, James told E! Online that she does not want to comment publicly on casting until the actors are chosen.
"It gets messy," she said. "You might end up with somebody else [than I suggested]."
James added that casting is a "collaborative effort" between herself and the film's crew and executives, but added that they have not yet had "discussions" about which rating the movie should aim for.
She also revealed that she "never thought about how [the writing] would translate to the big screen", adding: "I almost fell out of my chair when I got an email about film rights."
James recently revealed that she wrote sex scenes to the Black Eyed Peas track 'Sexy'.
Photo gallery - Fifty Shades of Grey, the rumoured cast in pictures:
Copyright: PA Images Jonathan Short/AP