"Suspect": I'm the Zodiac.
Inspector William Armstrong: And how did you kill your victims?
"Suspect": With a gun. No... with a hammer.
Dave Toschi: Hey, how do we know that this lead is real?
Inspector William Armstrong: It's very real. How do I know? Because I saw it on TV.
Paul Avery: Do you know more people die in the East Bay commute every three months than that idiot ever killed? He offed a few citizens, wrote a few letters, then faded into footnote... Not that I haven't been sitting here idly, waiting for you to drop by and reinvigorate my sense of purpose.
Robert Graysmith: Just because you can't prove it doesn't mean it isn't true.
Dave Toschi: Easy, Dirty Harry.
Robert Graysmith: I... I Need to know who he is. I... I need to stand there, I need to look him in the eye and I need to know that it's him.
Factual error: In a couple of scenes, most notably the one where Graysmith drives his kid to school, there are newer cars in the background than the year it is supposed to be. In this scene the car is a 1971 Chrysler, when the scene takes place in 1969.