The Danza: As signed on one of Boo's pictures, her real name is Mary after her voice actress Mary Gibbs.
Distanced from Current Events: The original punchline to gag of Harryhausen's getting "decontaminated" had the CDA blowing it up with explosives. A mere 60 days before the film's release, 9/11 happened, and it was hastily changed to the restaurant getting put under a force-field. The original scene can be found on the Blu-Ray features.
Doing It for the Art: The extensive work on the hair simulation for Sully is staggering, and the efforts clearly paid off.
Enforced Method Acting: In order to get her to laugh, Pete Docter would often talk to Mary Gibbs through sock puppets in the recording booth.
Jumped at the Call: Billy Crystal accepted the role of Mike because he felt bad for turning down the role of Buzz Lightyear on the first Toy Story movie.
Non-Singing Voice: In the German version of I Wouldn't Have Nothing If I Didn't Have You, Sulley's singing voice is provided by Thomas Amper, who also voiced George Sanderson.
Pop Culture Urban Legends: There's been a hoax going around associated with the movie. It involves a screenshot of a scene showing that Boo drew a drawing of her mother and "Uncle Roger" having sex. The picture is photoshopped. Despite this, people still claim it's a real case of Getting Crap Past the Radar or that it was on the first edition before later being censored.
From the obscure cartoon Pigs Next Door, John Goodman (Sulley) was main character Phil, while John Ratzenberger (the Yeti) and Jennifer Tilly (Celia)) were his neighbors the Stumps.
Throw It In!: All of Boo's lines. Obviously, then-toddler Mary Gibbs was not going to sit still in a recording booth and read her lines like a professional actress, so the team just follower her around the studio with a boom mic and recorded her playing. You can actually tell that she must have been in the Winnie-the-Pooh department, because one of her lines sounds like "I'm Tigger!".
Pete Docter's earliest concept involved an adult man with monsters as figments of his imagination.
Early drafts had Mike as Randall's scaring assistant and Sully as a non-Scarer factory worker.
Several other drafts didn't feature Mike at all, and were focused strictly on Sulley and Boo's relationship.
Another involved a Sully-prototype protagonist named Johnson teaming up with a human child named Mary and becoming Scarers together. The climax would have involved a Friend or Idol Decision, with Johnson wondering if keeping Mary's fixed door a secret was worth it to become Top Scarer.
Sulley was also going to have a land lady, probably a prototype for Roz.
Boo was originally going to be slightly older, around 6, but was made younger to make her more dependent on Sulley.
As shown by this clip, Mike wasn't originally going to have arms, and Sulley originally had tentacles instead of feet.
Sulley was supposed to have glasses, but that idea was scrapped due to concerns about them limiting his expressiveness.
According to the DVD's bonus features, Sulley (still with tentacles) was going to be Randall's assistant instead of Mike (who by this point has arms). At the same time, Randall was going to be called "Ned," a leftover of the original "Monster-Child scare team" treatment.
Sully's introduction after Waternoose mentions him in the opening scene was originally going to be him roaring at the camera and then seguing directly into the workout scene.
The scene at Harryhausen's originally had Mike trying to propose to Celia, which would have driven a wedge between him and Sully. However, it was quickly decided that Boo was enough of a source of conflict.
The original theatrical release was supposed to feature a THX trailer starring Shrek and Donkey, but it was pulled a week and a half before because Disney threatened to blacklist THX over the trailer, which wouldn't be seen for a couple of years after that.
A rejected concept was that Celia actually had the abilities of Medusa (hence having snakes for hair), having turned her ex-boyfriends to stone and putting extra pressure on Mike.
At least 31 levels (including bonus levels) were planned, but the game's Troubled Production resulted in the finished game only having 13 levels.
The tutorial level was originally going to revolve around Sulley and Mike going around a human campground, being taught how to perform moves by Flint (who actually has a completed but unused model in the game's files).
Sulley has unused animations for carrying Boo, implying she would have had a much larger role in the game.
Mike was originally intended to be a playable character instead of an NPC.
There's a lot of unused dialogue for conversations that don't happen in-game. Several conversations use placeholder dialogue from the movie, implying the original developers may have intended on getting John Goodman and Billy Crystal to reprise their roles as Sulley and Mike at some point.
Cutscenes of Sulley being shaved and frozen in carbonite are present in the game files and were intended to play if Sulley got caught by the CDA (in the game itself, Sulley merely dies and respawns).