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.
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 off-screen 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. Coincidentally, Disney would acquire Lucasfilm (the production company behind the Star Wars franchise, among others) for $4.5 billion in 2012, a year before the release of prequel Monsters University (2013)..
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.
Billy Crystal was originally offered the role of Buzz Lightyear in Toy Story (1995) and he declined, which he later regretted after seeing the film itself, as it would've been a great opportunity for him. They would later promise 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". Later in the late 1990s/early 2000s, Crystal's wife received a phone call from John Lasseter, who told him "Mr. Lasseter would like to speak to you", allowing him to voice Mike in Monsters, Inc. (2001).
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.
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.
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 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.
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.
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.
The restaurant that Mike and Celia are at is called Harryhausen's. 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 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.
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.
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.
News leaked in early fall of 2001 that this movie would feature the first teaser for Star Wars: Episode II - 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.
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.
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". His younger brother Joel Murray would later go onto voice Don Carlton in the film's prequel Monsters University (2013) 12 years later.
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.
At the beginning when Mike and Sully are walking to work Mike says "you've been jealous of my good looks since the 4th grade...", but the prequel Monsters University (2013) tells us Mike and Sully didn't actually meet until college.
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).
Originally intended in release on Thanksgiving in 2001 like the previous 3 Pixar Films, but was pushed forward 3 weeks to November 2nd to avoid clashing with the release of the live action film Out Cold (2001) by Touchstone Pictures on that same day.
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 name of the sushi restaurant where Mike is on his date is called "Harryhausen's." This is a direct nod to famed pioneer in stop-motion animation Ray Harryhausen, who worked on famed films like King Kong (1933) and Jason and the Argonauts (1963).
This film remains the only Pixar film where Daniel Gerson voices a character, being Smitty and Needleman, with him having passed away in 2016, though his last film worked on 16 years later being Cars 3 (2017) is dedicated to him. The only other film where he voiced a character is Big Hero 6 (2014), which is not by Pixar.
Pixar's only film where the logos are shortened at the start of the film, where they appear as seen in trailers (and outros to films in the case of the Disney Logo), pretty much to help accompany the music that plays during the title screen, but they do appear as normal after the credits.
This is Pete Docters only Disney/Pixar film that didn't win an Academy Award for Best Animated Feature. Up (2009) and Inside Out (2015) have both won, but Monsters, Inc. (2001) lost to Shrek (2001). This is also one of only two Pixar films to be nominated for the award without winning it, the other being Cars (2006).
Not only is the Pizza Planet truck by the trailer where Randall is getting beat up, but also the trailer that he is in resembles the same trailer from A Bug's Life (1998). So if the Pizza Planet truck is there too, than it made the same picture from A Bug's Life (1998) where the Pizza Planet truck is sitting by the trailer.
In the movie, there are several hidden Easter eggs referencing to what was there next film, Finding Nemo (2003). First, At the Harryhausen's restaurant, behind the octopus chef, there is a painting of Nemo on the wall. Second, when Sally throws Randall into a bedroom, a statue of Nemo can be seen on the wall into the bedroom. And last but not least, when Boo hands Sully some of her toys, she hands him a Nemo toy.
Running gags in the film include Mike being happy his face is in mass media despite his face being covered on TV and a magazine respectively, the CDA being informed of 2319 with George being the main victim, Boo causing panic among the monsters due to her being 'toxic' and Mike and Sulley attempting to return Boo back to her room but fail.
The first Pixar film to win an Oscar, being Best Original Song. It was nominated for 3 others, Best Animated Feature, Best Original Score, and Best Sound Editing, but lost them to Shrek (2001), The Lord of the Rings: The Fellowship of the Ring (2001), and Pearl Harbor (2001).
The monster in the opening scene doing the scare test is asked to confirm his name is "Mr. Bile" to which he replies that his friends call him "Phlegm" . Referencing that bile in the human body is commonly/casually referred to as phlegm.
The fancy restaurant that Mike takes Celia to, Harryhausen's, is reference to 'Ray Harryhausen' *qv) one of the earliest practitioners of stop-motion photography and claymation. Harryhausen is responsible for bringing a number of now-iconic monsters to life in films such as Jason and the Argonauts (1963), The 7th Voyage of Sinbad (1958)., and Clash of the Titans (1981).
At the start of the film Mr. Bile during the scare simulation scene causes comedy mayhem by suffering amusing injuries. This foreshadows the end of the film where instead of scaring kids the monsters instead make them laugh as it is apparently more powerful than scaring.
Sulley and Boo's relationship is possibly based on Baloo and Mowgli in _The Jungle Book (1967)_ and just like how Mowgli runs away from Baloo because he is forced to go to a man village Boo avoids Sulley after he accidentally scares her when doing a mock roar for Mr. Waternoose. Sulley is also similar to Baloo in appearance as he looks like a bear except he has horns and a long tail and is about the same height and bulk as Baloo. Plus 'John Goodman' voiced Baloo in The Jungle Book 2 (2003).
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).
The sequence where Sully thinks that Boo has gone through the trash compactor is an almost shot-for-shot remake (and an homage to) the Warner Bros Looney Tunes cartoon short Feed the Kitty (1952). In that cartoon a bulldog named Marc Antony mistakenly believes that Pussyfoot, a kitten he has adopted, has been made into a cookie.
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.
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).
The company play at the end of the film features Mike singing "she's out of our hair" whilst sitting on a stool this is a parody of the 1980 song and video to "She's out of my life" by Michael Jackson.
The signal for the CDA, 2319, may be a reference to the letters "W" and "S", which are the 23rd and 19th letters of the Latin alphabet. WS may stand for "white sock", a common threat to monsters, which can only be ridden by the CDA. W and S are also the initials of Mike and Sulley's last names (Wazowski and Sullivan).