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.
It normally took 11 to 12 hours to render a single frame of Sulley because of his 2.3 million animated hair strands (total number of hairs: 2,320,413). Part of that animation process was done via the use of moving "attractor / repeller" particles that functioned like virtual magnets . fans that would attract or repel the hairs proportionally over distance. By moving the particles along virtual paths they would provide a natural effect on the hair. An example of this could be seen in subsequent films such as the LOTR series, in which animated CG armies of thousands could be moved across a battle field, charging at orcs on the one hand and fleeing from trolls on the other.
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.
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.