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 was Ned/Randall's assistant.

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."

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. Also, the octopus behind the bar in the restaurant with only six legs is a reference to It Came from Beneath the Sea, a film in which Harryhausen created an octopus with six arms due to budget restrictions.

The Hidden Cafe is the name of a real restaurant in the San Francisco area which had been a favorite with Pixar's animators, including director Pete Docter, since the early '90s.

Pete's Barber Shop is named for director Pete Docter.

The original teaser (where Mike and Sulley enter a room in "Outer Magnolia" instead of Outer Mongolia) was specially made and features material which did not appear in the film. The scene from the trailer where Sulley tucks Boo into bed and says, "Don't worry, I'll protect you," does not feature in the film, either.

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.

In the background where the blob monster falls into a sidewalk grate, there is an art store called "Gallerie du Dominique". This store is named after Dominique Louis, an art director at Pixar.

On the scare floor leader board, the name immediately below Sullivan and Randall is Ranft, a reference to longtime Pixar writer Joe Ranft.

After Sulley says goodbye to Boo, he closes the closet door. When Boo jumps out of bed, she has grown 7% by the time she reaches the door. The programmers had to do this as she was too short.

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.

This is the fourth movie to feature both John Goodman and Steve Buscemi and is the first of the four not to be directed and produced by Joel Coen and Ethan Coen.

John Goodman pushed for Steve Buscemi to voice the villain Randall.

In early drafts, the character of Boo was written to be six years old. The writers decided to make Boo younger because it would make her more dependent on Sulley.

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.

It normally took 11 to 12 hours to render a single frame of Sulley because of his 2.3 million individually animated hair strands (Total number of hairs: 2,320,413).

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 arm-less with only legs.

In the German dubbing of the film, the Abominable Snowman speaks the Swabian dialect, which is mostly spoken in south-western Germany, and makes a reference to wishing he had banished to the Swabian Alps where he has friends.

The newspaper article concerning Boo's appearance in Harryhausen restaurant, though briefly seen, is actually readable and fully written in complete English.

Mary Gibbs (the voice of Boo) 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.

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.

Boo's teddy bear is the same bear as the one seen on the shelf in the simulator bedroom.

Billy Crystal was originally offered the role of Woody in Toy Story and he accepted. After Tom Hanks expressed a desire to voice the role, 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.