These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Alternate Character Interpretation: All over the place with the Zodiac. Does he believe everything he writes? Is he actually mentally ill and looking for help or is he just an amoral Attention Whore? Were Allen's reactions to seeing the cops and Robert a result of guilt or of being fed up with the harassment of accusations?
In Real Life, Marshall was proven to not be the Zodiac via fingerprint analysis.
One-Scene Wonder: John Carroll Lynch as Arthur Leigh Allen has a total of about five minutes screen time in a movie that's two and a half hours long, most of that time without dialogue. However, his interrogation scene is guaranteed to leave an impression on you and thanks to Lynch's mannerisms its easy to see why Allen was a prime suspect.
Similarly, Charles Fleischer, voice of Roger Rabbit, presides over one of the film's tensest scenes as the ambiguously creepy Bob Vaughn.
This Is Gonna Suck: Audience members unfamiliar with the particulars of the cases may have felt this when the woman with the baby got into the car with the stranger (possibly the Zodiac killer), particularly when he said "The more the merrier". Fortunately averted.
The Untwist: Toschi's number-one suspect was Leigh. And he was right...according to the film, anyway.
To save time David Fincher decided to not use bloody makeup for the murder scenes, thus all blood is CGI.
What an Idiot: Leigh, if the informant's story is true. He basically laid out his entire murder plot to him on New Years Day, as well as saying he would call himself the Zodiac.
The woman with the baby. Sure, trust some random guy to fix your car tires, and then get in the car with said guy after the tire he said he fixed falls off.Especially when it's widely known there's a serial killer on the loose.
Ensemble Darkhorse: Saurus is surprisingly popular despite being part of the Zodiac comic's Rogues Gallery. He even gets a side story dedicated to him that is set after the main story of the Zodiac comic. It helps that he's by far the nicest member of the villain team.