In the late 60s, a serial killer in Northern California began sending the press letters and crypto-grams. One cipher translated into a message that began: "I like killing people because it is so much fun." These unsolved murders roughly inspired 1971's Dirty Harry and other films, but the story of the Zodiac killer was ripe for a more faithful retelling. To deliver one, Fincher (Fight Club) corralled material from those who lived through the terror; the resulting account stars Downey Jr. as San Francisco Chronicle reporter Paul Avery, and Gyllenhaal as cartoonist Robert Graysmith, who wrote two books about the case.

During production, Fincher displayed his usual perfectionism, at one point doing 15 takes of Ruffalo, as detective David Toschi, handling a photo in a file. Downey Jr. says that Fincher afforded him some leeway, "but even that looseness was down a very, very thin road, because he's a director who holds the entirety of the action, the meaning, and the theme of the movie between those ears at all times."