The directors were aware of that line and tried out several scenes in which Sulley and Mike first met in grade school and then reunited in college. Everything they tried just distracted from the story they were trying to tell. On the advice of studio head John Lasseter, they dropped any attempt at explanation and hoped that viewers wouldn't remember the line or they would assume that Mike was speaking metaphorically ("you have been jealous of my good looks for a long time"). In a recent interview, the creators of Monsters University stated that "...since the fourth grade" is a typical phrase used by monsters. In this case, the joke would be that Mike is saying that Sulley has been jealous of his looks for a long time.

While adults' screams yield more energy, the adults are considerably more difficult to scare and far better equipped to retaliate. When recently scared children tell their parents about seeing monsters in their bedroom, their parents are most likely to dismiss these as nightmares generated from figments of the children's imagination. Scared adults, however, would not be so quick to dismiss their experiences with monsters as mere dreams or hallucinations; and some of them would be bold and curious enough to acquire weapons to attack the monsters, chase them back through the portal doors, and possibly even convince other adults to come with them to help them invade the monsters' world once they discovered its existence.

Only by putting considerable time and effort into setting up the scare were some monsters able to scare human adults in this movie, and this was strictly a one-time occurrence (ending in the door's destruction, which ensured no one would ever attempt it again). It was an act of desperation, as the monsters knew no other way to get back to Monsters University, and staying in the human world any longer would only increase their risk of being caught. Also, considering how toxic human children were considered to be, one can little doubt the monsters would believe human adults to be an even greater bio-hazard to them all; Dean Hardscrabble specifically described a monster's being caught by a child's parents as a failure that could have nigh-apocalyptically disastrous consequences for the monsters' entire society, and she did not seem to be engaging in hyperbole.

Billy Crystal's character Mike is actually the main protagonist, with John Goodman's character Sully as a strong secondary protagonist; the entire movie centers around Mike's struggle to break into the scaring industry, and how this ultimately led to his partnership with Sully. While both characters receive considerable development in this story, Mike is clearly the main character, and so his voice actor Billy Crystal likewise receives top billing.

Grammer was intending to voice Mr. Waternoose, but dropped out at the last minute. This forced the directors to write Mr. Waternoose out of the story and instead include him in a photo.