'Gone Girl' Author Talks Thrill of Success, Movie's Ending

"Gone Girl," Gillian Flynn's wildly popular dark thriller about a woman who disappears on her wedding anniversary, still has fans lining up nearly two years after it was first released.

The novel landed on The New York Times best sellers list for 79 consecutive weeks, and more than six million copies have been sold.

Movie rights for the page-turner - which delves into the murkier side of marriage - were quickly snapped up by actress Reese Witherspoon, and the film will star Ben Affleck.

The trailer went viral when it was released two weeks ago.

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In an interview with "Good Morning America's" Amy Robach, Flynn expressed her delight at the success her book has had.

"It was really wild because, first of all, the book did, what, better than anyone thought, so it became this kind of cool thing," she said. "And I was like, 'Oh, well, that's great. That's enough.'"

Then the movie happened, and she had a screenplay to write.

Flynn, who is married with a young son and a second baby on the way, said it was "pretty fun" to come up with the devious characters in her book.

Asked about rumors that the book's controversial ending might have been changed for the movie, Flynn said: "I cannot give away … the ending of the movie. … I wrote the screenplay, so it was in good hands.

"People, I think, were kind of worried that it would turn into, you know, a musical or something," she said, laughing, "the feel-good hit of the summer. And it's not that."

She said the film is as dark as the novel.

It's been a remarkable journey for Flynn. In 2008, she was laid off as a writer for Entertainment Weekly. A few years later, "Gone Girl" was on the cover of that same magazine.

"Gone Girl" was just released in paperback.

"It's so exciting," Flynn said. "It's this really wonderful thing when it goes out in the world and kind of takes on its own life."

The movie opens in October. Another of Flynn's books, "Dark Places," is being made into a film starring Charlize Theron.