Fans of Gillian Flynn's novel Gone Girl are set to be in for a surprise as she has written a completely new ending for the film adaptation starring Ben Affleck.

The David Fincher directed film will have a completely new third act according to Flynn and she told Entertainment Weekly that Affleck was shocked by the decision.

"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 continued, "There was something thrilling about taking this piece of work that I'd spent about two years painstakingly putting together with all its 8,000,000 LEGO pieces and take a hammer to it and bash it apart and reassemble it into a movie."

Gone Girl tells the story of a dysfunctional marriage between a man, played by Affleck, and his wife who goes missing, played by Rosamund Pike. How I Met Your Mother's Neil Patrick Harris also stars in the film, which is slated for release on October 3.