Fans of Gillian Flynn's best-selling novel Gone Girl are in for quite the third act surprise when the book hits theaters in October.

The author wrote an entirely new ending for the movie adaptation of her book starring Ben Affleck and Rosamund Pike.

"Ben [Affleck] was so shocked by it. He would say, 'This is a whole new third act! She literally threw that third act out and started from scratch,'" Flynn said in an interview with Entertainment Weekly, published in the magazine's latest issue.

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Flynn had no problem changing her ending and had in mind who she wanted to direct the film adaptation while writing it.

"When I was writing Gone Girl there were certain parts where I thought, 'David Fincher would really kill this scene,' " Flynn said of The Girl with the Dragon Tattoo visionary. "I thought he'd inject just the right sense of necessary malice," said Flynn.

The crime novelist and director will work together to put a brand new mystery on the big screen. Fans will get all the characters they know and love, while being able to leave the theater completely surprised.

In the same interview, Flynn outlined why she believes Affleck and Pike were the best choices to play the lead characters Nick and Amy Dunne.

"Ben knows inherently what that experience is like. He knows what it's like to be hunted," he said. "You have to wonder if Nick did horrible things to his wife, and you have to see him do some really not good things, but at the same time you have to able to say, 'I'd like to have a beer with that guy."

Flynn, Fincher, Affleck, and Pike will all put their talents together in what is setting up to be a thrilling ride. Gone Girl is set to release in theaters Oct. 3, 2014.