Like No Country for Old Men, David Fincher's latest is much more about the experience of watching the film than about anything that the movie ostensibly markets itself as. It's certainly not a serial killer film, for instance. While many critics have pointed out that this is a dramatic turn away from his first great movie, Se7en, they rarely note that Zodiac is its antithesis for any other reason than generic expectations alone (the latter film is not a horror film, they posit, and I personally am not really convinced that the former is either). In any case, Se7en concerns the actions of a killer whose murders are ciphers from God, all existential subtext circulated in the characters' philosophical conversations about what the murders mean; it's something of a fable on the order of Camus's Myth of Sisyphus, as the detectives realize the futility of saving a world that has become a kind of Hades. Zodiac, on the other hand, is entirely about the ways in which the murders are objects to be examined not for their content but for their context, a remove from the earlier film which allows the detectives and reporters on this case to realize that the killer's ciphers are not important for their meaning so much as they are important for how they fuck with the entire nation's infrastructure.
The film as a whole embodies this kind of investigative approach more reminiscent of social sciences than it is of literary criticism (note the contrasting motifs of the two films between the library and the movie theatre, between the location as a space in which knowledge is kept and the location as a space in which people live and act upon each other). The characters never contemplate the meaning of the crimes, but rather treat the entire affair as a series of actions and events with particular details to be used as evidence in relation to the larger social context from which they emerged. The killer works so well as a metaphor here because he has no referent, and, as a result, the existential dread is shifted from iconography and symbolism to the mundane chatter of the everyday that is used to maintain the illusion that our social networks cannot be breached as easily as the Zodiac killer demonstrates. In other words, the film has shifted from the idea of cinema as an object in which meaning is represented to us and toward the philosophy of cinema as a series of moments and spaces which present themselves baldly to a viewer. That it makes this clear through its own obsessive focus on period detail and its painstaking attempt to make everything relative to how people experience the sights and sounds of that period (for instance, a different actor portrays the Zodiac at different moments to match the descriptions offered by witnesses) makes this film an incredibly sophisticated statement about the cinematic experience.