The Danza: As signed on one of Boo's pictures, her real name is Mary after her voice actress Mary Gibbs.
Descended Creator: Pete Docter voiced the CDA agent giving them the all-clear to decontaminate Harryhausen's.
Distanced from Current Events: The original punchline to the 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 Sulley is staggering, and the efforts clearly paid off.
Enforced Method Acting: Because Mary Gibbs was so young when she recorded her parts as Boo, Pete Docter would often talk to Mary Gibbs through sock puppets to get the necessary emotions from her. Other times, she'd be given money or candy to get a happy reaction and have it taken away to get anger and sadness. Sometimes her mother would come in and tickle her or even pick her up and toss her in the air to get her to laugh into the microphone.
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 that it was on the first edition before later being censored.
Many articles and sources claimed that Boo was singing the titular song from Beauty and the Beast while in the bathroom stall. While the melody is somewhat similar, Mary Gibbs confirmed in a Reddit AMA that the lyrics are mainly nonsense.note When she was first asked to sing a song for the crew, she originally sang "Wheels on the Bus", but this was unused due to it being copyrighted.
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 any lines like a professional actress, so the team just followed 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, Each monster represented a fear he had, and conquering those fears caused the monsters to eventually disappear.
Early drafts had Mike as Randall's scaring assistant and Sulley 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 Sulley-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.
In this same treatment Sulley was also going to have a land lady, probably a prototype for Roz.
Similarly, Sulley would have ended up as The Exile in this treatment, with Mary dropping her door on him to save him from the cops and Ned. Because of his exile, Sulley and Mary would grow old together, well into their senior years, later repurposed into Up during the "Married Life" sequence.
Waternoose was to have a design slightly resembling Gillman, all while still having the suit and multiple eyes that carried all the way to his final, crab-like design. In concept art of this same design, Waternoose was also shown to be a Cigar Chomper.
Randall's original design had him as an orange cyclopean, fishlike monster who would have acted as a mentor to Sulley, before becoming a Green-Eyed Monster from his recent string of successes alongside Mary, later leading the monster hunt for the two of them once he finds out about his secret.
Boo was originally going to be slightly older, around 6, but was made younger to make her more dependent on Sulley.
Sulley was supposed to have glasses, but that idea was scrapped due to concerns about them limiting his expressiveness.
Early concept art of Randall variably depicted him with a bowtie and/or a waistcoat.
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.
In early development, the city of Monstropolis was designed with a more gothic and sinister aesthetic, similar to Halloween Town from The Nightmare Before Christmas. When later drafts established that being scary was just a job for the monsters rather than their natural behaviour, the city was redesigned with a stronger resemblance to an average human town.
Mike was going to propose to Celia at Harryhausen's before Sully interrupted. It was cut because it would make his relationship even more strained. Funny enough, in the DVD commentary, the directors forgot that reason and had to call Dan Gerson to exlain why.
Sulley'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 Sulley. 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.
Ted's original roar was that of a "famous movie monster" according to Pete Doctor on the audio commentary, most likely Godzilla. Unable to get the rights, they swapped it out for the sound of a chicken. An Early Draft Tie-In of this idea can be found in one of the TV spots prior to the film's release, where Ted lets out a low roar when stepping up to home plate.
Due to Pixar not renewing its seven-film distribution deal with Disney in 2004 that would also give the latter full rights to its properties and sequel production, a sequel called Monsters, Inc. 2: Lost in Scaradise was intended to be produced by a newly established studio called Circle 7 Animation, along with Toy Story 3 and a sequel to Finding Nemo. The plot would have involved Sulley and Mike visiting Boo's room to celebrate her birthday, only to find an old woman living where her room used to be. The two would then decide to explore the human world in search of her, only to get lost. With the deal falling through in January 2006, and then-CEO Michael Eisner, who Pixar seriously disdained, getting ousted and being succeeded by Bob Iger, Disney would purchase Pixar and give creative control back to them, with the sequel being scrapped in favor of Monsters University. Circle 7 would be shut down two months later, with most of the animators going to either the feature animation department or DisneyToon Studios.
Working Title: "Hidden City" was its code name, and "Monsters" was the name for an early script treatment.
Fan Remake: Project 2319, a remake of the original game that plans to restore all the cut content and add in new features.
Troubled Production: The game's development was very hectic, not helping the fact that the developers went defunct at the end of 2001, forcing some of the staff members to finish the game up from home. This resulted in a plethora of content that was cut from the game.
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).