Let me begin explaining this:

ZODIAC has duration of 160 minutes or 2 hours and 40 minutes. - Is about the investigation of a serial killer that can only be demonstrated three killings even when he claims 13 or more. - The killer was never caught - The movie is based on real events


At almost three hours ZODIAC could have been a decent four night's miniseries in any network channel. The three killings are strong but not really gory. It does not have "A" list actors even when the acting is excellent.

Even if you do not know the story (as it was my case) after the first half, there not much to sustain any interest. All you get are continuous dialogs, a little humor and more dialogs.

Robert Downey Jr character is the only one interesting (but partially developed). No other really deserves real interest or rooting for.

In brief; I am sure there are better ways to spend almost three hours of your life.