as "The Game" or "Se7en" David Fincher on board.
It's a thriller in the very broad terms, it moves so slowly and patiently. You slowly become obsessed alongside the film's main character; Robert Graysmith. Graysmith exist as a stand-in, ultimately demonstrating a evolution of curiosity to obsession that eventually. The audience is Graysmith; and it's required for the audience to have so much culpability into the Zodiac case.
As fore-mentioned before, Zodiac is not this fast-paced action-drenched thriller, it's so patient and calculated. The way it requires the audience to just be involved in it and then leaves it hanging at the end is just so breathtakingly refreshing and completely resonates with you that it forms into one of the best conclusion brought to film. But part of the reason the conclusion is so gosh damn perfect, is just how imperfect it is. It cannot give it's audience a firm ending so they can go home and sleep properly at night, and you are frustrated by the lack of a solid conclusion and the fact that you'll never know the solid truth, but that's the truth of life sometimes and Fincher boldly takes you to that place.