NYT:
But the source of his dark-hued lens on life, Mr. Fincher suggested, might be as simple as that original bogeyman. “It was a very interesting and weird time to grow up, and incredibly evocative,” he said. “I have a handful of friends who were from Marin County at the same time, the same age group, and they’re all very kind of sinister, dark, sardonic people. And I wonder if Zodiac had something to do with that.”
Mr. Fincher was first approached about “Zodiac” by Brad Fischer, a producer at Phoenix Pictures, with a script by James Vanderbilt. It was based on two books by Robert Graysmith, a former San Francisco Chronicle cartoonist who became obsessed with the Zodiac, and who built a case against one suspect, now dead. Mr. Fincher said he wanted Mr. Vanderbilt to overhaul the script, but wanted first to dig into the original police sources. So director, writer and producer spent months interviewing witnesses, investigators and the case’s only two surviving victims, and poring over reams of documents.
“I said I won’t use anything in this book that we don’t have a police report for,” Mr. Fincher said. “There’s an enormous amount of hearsay in any circumstantial case, and I wanted to look some of these people in the eye and see if I believed them. It was an extremely difficult thing to make a movie that posthumously convicts somebody.”
Here.