Actor Allusion: Peach, played by Allison Janney (more famous for playing C.J. Cregg on The West Wing) announces that the dentist is about to do a root canal, bringing to mind the hilarious West Wing episode "Celestial Navigation".
There are a couple of allusions (the song "Beyond the Sea" and the line "That's my boy.") to Albert Brooks in My First Mister
The Other Marty: William H. Macy did a complete voiceover job as Marlin, but the brain trust felt the character needed someone more in line with Marlin's ever-manic and worrying personality, and recast the role with Albert Brooks. According to director Andrew Stanton, it "saved the movie".
The Red Stapler: After the release of this movie, kids wanted clownfish and others seen in the movie. Ironic, considering half the movie involves Nemo wanting to escape life in a tank.
What Could Have Been: A deleted scene had Nemo making it through the sewage treatment plant. While this would have explained the question raised by the scene's absence, it was cut, possibly for being too frightening.
According to the DVD commentary, the director cut the scene for two reasons: First, it put too much more focus on Nemo- as Marlin is the main character of the film. Second, it didn't add anything that the rest of the movie hadn't already done; making it just a repeat of the filter scenes.
Gil was going to lie to Nemo about his life in the sea in order to look more impressive. Nemo would then find out that Gil was lying when one of the patients starts reading a storybook about pirates that shared the exact same details. This subplot was cut because it made Gil too unlikeable.
The death of Nemo's mother was supposed to be slowly revealed in flashbacks, and was part of the initial structure of the movie, but was scrapped as the director felt that the audience would catch on what would happen fairly quick and thus, they would be stringing it out longer than necessary; starting the movie with her death would immediately justify Marlin's overprotective parenting. Reportedly, this plot structure was one of the reasons Michael Eisner felt Nemo would bring Pixar back to Earth.