Defictionalization: The Blu-ray comes with an Amazing Amy book ("Amazing Amy The Tattle Tale"). It's every bit as creepy in its over-the-top portrayal of Amazing Amy as the real Amy claims it was.
Dawson Casting: Ben Affleck is actually nine years older than Carrie Coon, despite playing twins. He was actually concerned about the age difference, and decided that the twins should be thirty years old, to make it more believable.
Doing It for the Art: Ben Affleck postponed directing Live by Night in order to work with David Fincher, even stating, "He's the only director I've met who can do everybody else's job better than they could." On-set one day, Affleck changed the lens setting on a camera an almost indiscernible amount, betting a crew member that Fincher wouldn't notice. Affleck lost the bet as Fincher brought up, "Why does the camera look a little dim?"
Rosamund Pike gained and lost thirteen pounds three times to play the character at different times in her life. She stuffed herself with hamburgers and malts to gain the weight, and exercised with a professional boxer for as long as four hours a day and ran five miles (in forty-two minutes) to lose the weight.
Kim Dickens, normally blonde, became a brunette to play Detective Boney.
Inspiration for the Work: The author said she was inspired to write the story after hearing about the Laci Peterson case, where the handsome, seemingly perfect husband of a heavily pregnant woman was shown to be curiously unfeeling and calm regarding the disappearance of his wife. Turns out he was having an affair, too, and attempted to continue the affair all throughout the search efforts. The difference is that the husband was eventually convicted of murdering his wife, while Amy was actually the mastermind behind her disappearance.
Lying Creator: Fincher and Flynn claimed that the film would feature a reworked third act. The film's third act, up to and including its ending, is almost entirely faithful to the book.
Throw It In!: Rosamund Pike said in a magazine article about making the movie that she including some levity during a scene where she's making pancakes that wasn't in the script, in order to convey a light-hearted friendliness to Ben Affleck. Pike said that the nature of Nick and Amy's relationship didn't allow or call for much (if any) playfulness, but she wanted to add it because it could play as either genuine and showing some basic affection between Nick and Amy, or it could be another part of Amy's warped psychology relative to everything else she says and does in the film.
Wag the Director: Originally, Nick was supposed to wear a Yankees cap in one scene. But Ben Affleck, a die-hard Red Sox fan, refused to wear it. Ultimately, he and David Fincher compromised, and he wears a Mets cap.