Mary Gibbs was so young that it proved difficult to get her to stand in the recording studio and act her lines. Instead, they simply followed her around with a microphone and cut Boo's lines together from the things she said while she played.
About 3:26 into the movie, when the simulation is ended and the monster reaches for a knob on the control panel to review the videotape, just below and to the left of the knob is a little indicator which reads "510-752-3000", which is Pixar's phone number.
Disney/Pixar prepared a special trailer for Monsters Inc. to show before Harry Potter and the Sorcerer's Stone (2001). Mike and Sulley play a game of charades in their apartment, with Sulley acting out "Harry Potter." Mike has difficulty solving the puzzle, some of his guesses being Dirty Harry (1971), "Harry Flowerpot" and "When Hairy met Sulley," (a reference to Billy Crystal's starring role in When Harry Met Sally... (1989)). Eventually Sulley puts on round spectacles, sticks a paper lightning bolt to his forehead and sits on a broom with an owl on his arm, to which Mike ecstatically guesses The Sound of Music (1965). Sully gives up and walks offscreen as Mike finally guesses "Harry Potter" and the cut away to title cards noting that "Monsters Inc." is "Now showing at a theater near you. Really near you. Like, maybe, right next door." At the end of the trailer, it is Mike's turn to act out a part, with a very bored looking Sully guessing it is Star Wars: Episode IV - A New Hope (1977) within seconds, much to Mike's dismay.
Billy Crystal was originally offered the role of Buzz Lightyear in Toy Story (1995) and he declined. They offered Crystal a future role in a Pixar film that they would tailor to his talents. Crystal told them to "do what's best for your movie and don't forget me when something good comes around." They didn't and offered him the role in this film.
When Sully greets Ted (the monster so big we only see the lower half of his body), Ted was originally supposed to roar like Godzilla. The filmmakers asked permission from Toho (Godzilla's film studio) to use a Godzilla roar, but Toho turned them down, so Ted responds with a chicken cluck instead. In one of the credit bloopers, though, he was revealed to be a dinosaur - Rex, from Toy Story (1995) - auditioning for a role in the movie.
Jennifer Tilly, who plays Celia Mae, was married to Sam Simon who was one of the creators of The Simpsons (1989). She raved to the Pixar crew about a script she read written by a Simpsons director for an animated film. The Director was Brad Bird and the script was for The Incredibles (2004) which became a Pixar film 3 years later.
Boo's real name is Mary, as shown briefly on one of the crayon drawings she shows to Sulley in the scene where Boo is going to sleep on Sulley's bed. The actress who provided the voice of Boo is Mary Gibbs.
The animators considered giving Sulley tentacles instead of legs at one point, and glasses at another. However, they decided to use legs because they believed the audience would concentrate more on the tentacles than Sulley's face. They also considered making Mike armless with only legs.
The restaurant that Mike and Celia are at is called the Harryhausen. This is an homage to Ray Harryhausen, the man who made the stop-motion animation monsters for films like Jason and the Argonauts (1963). Also, the octopus behind the bar in the restaurant with only six legs is a reference to It Came from Beneath the Sea (1955), a film in which Harryhausen created an octopus with six arms due to budget restrictions.
In the first scene, one of the toys on the boy's bedroom shelf is the toy plane that hung from the ceiling and caused Buzz to "fly" in and as Mike and Sully go through the scare floor, the rolling clown, from Toy Story (1995) can be seen in the background. Also, when Randall is practicing camouflaging into backgrounds of walls, one of the images given to him is the wallpaper Andy had in his room.
In the original draft of the script, Sulley was not to be a scarer but a worker named Johnson with brown fur. In fact, a quite long storyboard of the Sulley-as-lowly-helper is shown on the DVD. Also, Mike was the assistant of Randall (who was named Ned in the first version). In another draft, Sulley, originally called "Barrymore", was Ned/Randall's assistant. George Sanderson (the monster who keeps getting caught by the CDA for "2319" emergencies) is the monster who was supposed to be the protagonist for the original Monsters, Inc. concept - a bumbling, inept monster who couldn't scare anyone, 'til a timid girl, put upon by her brothers, teaches him how to be scary. George in the final version is even the same color and monster type in the original storyboards.
John Goodman and Billy Crystal sometimes recorded their lines in the same room together, an unusual move for animated films, where actors more often work alone. Steve Buscemi and Frank Oz (Randall and his assistant Fungus) also recorded their lines together for the bathroom scene.
At Harryhausen's Sushi Restaurant, employees shout, "Get a paper bag!" whenever somebody walks through the door. In Japan, it is customary for employees of any store to shout "irasshaimase!" (pronounced ee-rah-shai-mah-seh"). The monsters shouting, "Get a paper bag!" is a phonetic reference to this fact. There's a similar Japanese reference in Toy Story 2 (1999) (see Trivia).
In the German dubbing of the film, the Abominable Snowman speaks the Swabian dialect, which is mostly spoken in southwestern Germany, and makes a reference to wishing he had banished to the Swabian Alps where he has friends.
According to the commentary, when Mike is on the run from Randall, he hides in a room but relinquishes himself when he turns and sees Randall in a window, staring at him and beckoning. It is only when Mike gives himself up that he realizes Randall was in the opposite room, looking in the mirror and picking his teeth.
News leaked in early fall of 2001 that this movie would feature the first teaser for Star Wars: Episode II - The Attack of the Clones (2002). Hundreds of Star Wars fans paid admission for the movie just to see the teaser and reportedly some left after seeing it.
Bob Peterson, the movie's story supervisor, provided the temporary voice of Roz, the green secretary, during production. The nasal, sing-song voice proved to be such a success that they kept it in the final film.
Bill Murray was considered and tested for the role of Sulley, but the director, Pete Docter, said that when the filmmakers decided to offer it to Murray, they were unable to make contact with him and took that to mean "no".
On the DVD bonus features, the three ads at the end of the company play, "Put That Thing Back Where It Came From, Or So Help Me!" are for Tony's Grossery (seen earlier in the movie when Mike and Sulley pass it), Pixar Shorts ("We've got the important things covered") and Pete's Barber Shop (specializing in/selling Claw Wax, Scale Polish, and Mange Management).
The controls for the Monsters Inc. door mechanisms include a button that starts the process labeled "FIZT". At the time, Fiz-T (Physics-Tool) was the latest software developed by Pixar Animation Studios to realistically render complex physical models - notably in this film to model Sulley's fur and Boo's clothing. Another button, "IKT," was the name of Pixar's "Inverse Kinematics Tool."
When Sulley is about to say goodbye to Boo, Boo is trying to get him to play. A doll of Nemo appears. This is the first appearance of the character who would be the focus of Finding Nemo (2003), storyboarded before this movie but not completed until two years later (Marlin also appeared here in a portrait behind the Harryhausen chef and Nemo decorated the wall of a room Randall was thrown into). Boo also hands Sulley a Jessie the Yodeling Cowgirl doll, which first appeared in Toy Story 2 (1999) and the Pixar ball originally from Luxo Jr. (1986).
Randall Boggs is last seen in the trailer home from A Bug's Life (1998), next to the Pizza Planet truck from Toy Story (1995). The man inside the trailer is the same model as the simulated boy from the first scene. The mom is the same boy but with a "bun" placed on his head.
Towards the end of the movie, Mike opens a package with a magazine featuring Sulley and Mike on the cover. On the back is a full page advertisement for a computer system with tagline at the bottom "Scare Different..." Meanwhile, the new slogan "Think funny" appears all over the scare floor after the change in the corporation. These are both nods to Steve Jobs, CEO of Pixar and Apple (which had the "Think Different" ads).
In the ending sequence when the monsters make children laugh instead of scream, one monster on the factory floor can be seen carrying a sledgehammer and a watermelon - a clear homage to the comedian Gallagher, who popularized that particular gag.