Bob Peterson, an artist and director at Pixar, has voiced a number of characters in Pixar's films. Before animation begins on a film, the entire movie is story-boarded and then the story boards are cut together into a rough version of the film. Often the filmmakers will voice the characters themselves just to get an idea of how the film will play. Bob was so good during some of these rough cuts that they didn't bother trying to find a voice actor to replace him. Thus far he's been Roz in Monsters, Inc., Mr. Ray in Finding Nemo, and Dug and Alpha in Up.