1. Pixar conceived the idea for the films Monsters Inc., Finding Nemo, A Bug's Life, and Wall-E during one lunch meeting in 1994.
2. Iconic Muppet performer Frank Oz, came up with the name Mike Wazowski.
3. Bill Murray was the first choice to voice Sulley. He reportedly tested for the role and was interested, but when the producers were unable to make contact with him after the test, they took that to mean “no.”
4. Boo was originally written to be six-year-old. She was rewritten to be toddler because the writers felt it would make her more dependent on Sulley.
5. In the original script, Sulley was a janitor and not a scarer. Also, Mike’s character did not exist.
6. There are over 2.3 million individual strands of hairs on Sulley.
7. Because each individual strand of hair had to be animated, it took an average of 12 hours to render a single frame of Sulley.
8. Boo's real name is Mary.
9. This was the fourth movie that John Goodman and Steve Buscemi costarred together in.
It was also the first film they costarred together in that was not directed or produced by the Coen Brothers.
10. The song Boo sings while in the restroom is "Beauty and the Beast."
11. The scene where Sulley thinks that Boo has been crushed in a trash compactor, is a direct reference to the Looney Tunes short Feed the Kitty.
Sulley even mimics the same reactions, shot-for-shot, as Marc Anthony the protagonist of Feed the Kitty.
12. Roz was voiced by Bob Peterson, who also did the voice of Dug in Up.
Bob also co-directed Up.