Pixar's characters are often planned years in advance. Nemo first appeared as a stuffed toy on a couch in Boo's room in Monsters, Inc. (2001). This movie introduces the main characters of post-2003 Pixar films. A boy in the dentist's office is reading a "Mr. Incredible" comic book, anticipating The Incredibles (2004). Luigi the car is driving by the dentist's office, anticipating Cars (2006).
To see how realistic they could make it appear, the art team were asked to make exact copies of actual underwater and above-water shots. Ultimately the results were simply deemed "too realistic" for a cartoon.
Andrew Stanton pitched his idea and story to Pixar head John Lasseter in an hour-long session, using elaborate visual aids and character voices. At the end of it, the exhausted Stanton asked Lasseter what he thought, to which Lasseter replied, "You had me at 'fish'."
Rendering a frame which lasted about 1/24th of a second in the film could take up to four days because of the complexity of the underwater environment with sunlight coming through the water and hitting fish scales.
Film makers were worried that comedienne Ellen DeGeneres might not be able to perform the dramatic scene where Dory begs Marlin not to leave, so at the end of a day of recording other scenes asked her to record a trial reading of the scene with the intention that she go home with a recording of it to work on her actual performance. DeGeneres agreed, but her trial reading was so heartfelt and emotional that (apart from a few small edits) this is what was used in the final cut of the film.
Dory does get Nemo's name right a total of 7 times, 4 of them without being corrected first. Of course some of these times are in quick succession. She also calls him Chico, Fabio, Bingo, Harpo, and Elmo, in that order.
Director Andrew Stanton originally planned to reveal the fate of Marlin's wife gradually through flashbacks seen periodically as the story unfolded. After a few early in-house screenings, he found that Marlin came off as too much of a worrywart, and decided to reveal the entire back-story up front, thus making Marlin more appealing by establishing the reason for his over-protectiveness.
Director Andrew Stanton did the voice of Crush the sea turtle. Stanton never intended to do the voice of Crush, only providing the voice for the film's rough cut, but when it proved popular in test screenings, he decided to do it for the final film. Stanton recorded all of Crush's dialogue lying on his couch in his office.
Squirt the sea turtle is voiced by Nicholas Bird, the young son of Pixar director Brad Bird. Director Andrew Stanton was inspired to cast Nicholas when Brad Bird was showing home movies around the Pixar offices.
The look and feel of the underwater world was essential to the film's success. To that end, the production crew were all exposed to visits to aquariums, diving stints in Monterey and Hawaii, study sessions in front of Pixar's own 25-gallon fish tank and even a series of in-house lectures from an ichthyologist.
Afraid that kids would try releasing their pet fish by flushing them down a drain, a company that manufactures equipment used by water filtration and sewage treatment plants released a warning the Thursday after the film came out saying that, even though drains do eventually reach the ocean, before it got there the water would go through equipment which breaks down solids, and went on to say that in real life the movie would more appropriately be called "Grinding Nemo".
Megan Mullally revealed that she was originally doing a voice in the film. According to Mullally, the producers were quite disappointed to learn that the voice of her character Karen Walker on the television show Will & Grace (1998) wasn't her natural speaking voice. The producers hired her anyway, and then strongly encouraged her to use her Karen Walker voice for the role. When Mullally refused, she was fired.
Albert Brooks was always Andrew Stanton's first choice to voice the part of Marlin. Although Brooks had done several episodes of The Simpsons (1989), he found voice work for a feature length cartoon to be substantially different in that he had to do it in isolation, and not alongside any other actors. He didn't particularly enjoy the experience.
Nemo's father Marlin was originally voiced by William H. Macy. According to James Stewart's book "DisneyWar", it was after seeing an early cut of the film with Macy's voice that then-Disney CEO Michael Eisner infamously told his board of directors, "This will be a reality check for those guys...It's OK, but nowhere near as good as their previous films. Of course, they think it's great. Trust me, it's not." Director 'Andrew Stanton' recast the role of Marlin with Albert Brooks, and the film went on to get some of Pixar's best reviews ever and become the highest-grossing animated film of all-time.
Per the DVD extras, Albert Brooks spent an entire day in the recording studio improvising badly mangled versions of the anemone joke; no two tellings were fumbled in the same way. He had the recording technicians in stitches for the duration.
Though never mentioned in the film, it is revealed by the directors in the commentary that Crush and his crew of thrill-seeking turtles are headed for Hawaii. Also mentioned in the commentary is that the young turtles' shells are modeled after Hawaiian shirts.
According to the DVD, the names of the nine boats seen in the Sydney harbor are: Sea Monkey, Major Plot Point, Bow Movement, iBoat (a reference to iTunes, the company created by Pixar CEO Steve Jobs), Knottie Buoy, For the Birds (2000), Pier Pressure, Skiff-A-Dee-Doo-Dah (a reference to Song of the South (1946)'s most famous song "Zip-a-Dee-Doo-Dah"), and The Surly Mermaid.
In the original cut of the movie, the whale that swallows Marlin and Dory approaches them from the front. This version of the scene appeared in an early trailer. The final version, with the whale coming from behind, was inspired by an early animation test showing a whale emerging from the murk of the ocean behind a small fish.
According to the DVD, there are some references to Massachusetts in the film as one of the creators is from Rockport, Massachussetts. While various sea creatures are relating Marlin's quest across the ocean, one step is a group of lobsters that speak with Boston accents and slang ("wicked daahhk").