Early design of Sully and 8-year-old Boo.
- Bill Murray read for the role of Sulley before John Goodman was cast. However, Pete Docter couldn't get back in touch with Murray after his screen test, which he took as a "no".
- Robin Williams, Eddie Murphy, Jack Black and Drew Carey were considered for the part of Mike before the casting of Billy Crystal.
- Jim Carrey, Chris Barrie, Vincent D'Onofrio and Jeff Goldblum were considered to voice Randall before Steve Buscemi was cast.
- Patrick Stewart had discussions for the role of Waternoose before the casting of James Coburn.
- 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.
- The DVD and Blu-ray releases have a story treatment
narrated by Pete Docter. Among its many differences:- A Sulley-prototype protagonist named Johnson teams up with a human child named Mary and they become 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.
- Mary had nine older brothers who prank and scare her to no end, making her wish she could scare them for a change. This results in her becoming Johnson's secret scare partner after he sees how pathetic he is at scaring.
- Johnson was going to have a landlady, probably a prototype for Roz.
- Johnson 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 to his final, crab-like design. In concept art of this same design, Waternoose was also shown to be a Cigar Chomper.
- The original design for Randall (named "Ned" in this treatment) 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.
- The film originally had a framing device that served as a backstory for the Monster World.
- In the CGI test included in the DVD extras, Mike wasn't originally going to have arms, instead being able to emote in a similar way using his legs. Proto-Sulley (still named Johnson at this point) originally had tentacles instead of feet, and had a pink, more boxy design.
- 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 still going to be called "Ned," a leftover of the original "Monster-Child scare team" treatment.
- Sulley's original concept art made him look a lot weirder◊. Later art◊ was closer to his current design.
- 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 behavior, 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, and that Boo was enough of a source of conflict. Funny enough, in the DVD commentary, the directors forgot that reason and had to call Dan Gerson to explain 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 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 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 before the film's release, where Ted lets out a low roar when stepping up to the 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 a very different 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.
