Here's what director Flincher said about the film at a Tribeca Festival Panel with Steven Soderergh:
“There was so much flak after Zodiac came out about people saying, ‘Why didn’t you go down this rabbit hole? Why did you only go down the Graysmith rabbit hole?....That’s the book that we bought,” he said, cutting off his own line of questioning. “We didn’t buy everyone’s book about the Zodiac.”
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The film is based on non-fiction books (Zodiac and Zodiac Unmasked) by Robert Graysmith, the newspaper cartoonist. The movie was written by James Vanderbilt. The Film stars Jake Gyllenhaal, Robert Downey Jr., Mark Ruffalo, Chloë Sevigny, John Carroll Lynch and Anthony Edwards.
Fincher made his comments in response to a fan question about making projects involving real people, including ones who are still alive, like the subjects of his Facebook origins film, The Social Network, and the inspirations behind his Mindhunter series:
“You have a responsibility to make sure that you are saying what you want to say because chances are they can deck you in an airport. So you want to be conscious and be smart about it. Making movies about things that are ripped from headlines is a slippery slope. I think it’s important to be responsible, and by the same token, you also have to entertain an audience.”
Source: TheHollywoodReporter