- 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?
- Award Snub: No Oscar nominations whatsoever.
- Fan-Disliked Explanation: Who was Rick Marshall? Maybe he did kill people and film it, but you'll never know!
- Hilarious in Hindsight: Robert Downey Jr. and Mark Ruffalo are both in this film, they trade barbs, they bicker. We see them do it again in the Marvel Cinematic Universe.
- The third lead is Jake Gyllenhaal, who almost replayed Tobey Maguire in Film/Spiderman2. And now Spider-man is coming to the MCU (not played by Gyllenhaal though).
- 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.
- Paranoia Fuel: Be careful when someone pulls up next to you.
- Lampshaded when Robert briefly thinks that Bob Vaughn is the Zodiac.
- The Untwist: Toschi's number-one suspect was Leigh. And he was right... according to the film, anyway.
- Visual Effects of Awesome: Certain scenes featuring San Fran to be digitally created and you won't even know which ones.
- 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.
- Crazy Awesome: Libra, as might expected from an Ascended Fanboy.
- 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.
- Nightmare Fuel:
The Deus Ex mod:
- Goddamn Bats: Large numbers of Grays keep spawning in the final areas, despite your ammo being limited.