Mary Gibbs provided the original English voice of Boo in Pixar’s Monsters, Inc.
Gibbs was a child actor during production, and the filmmakers recorded her dialogue in an unusually informal way. Rather than having her read every line from a script, the crew often followed her around and recorded spontaneous reactions, then shaped Boo’s performance from those sounds.
Boo is a human child who enters Monstropolis after following Sulley through a door. Because she is very young, much of her communication consists of short words, laughter, and expressive noises rather than long dialogue.
A common mix-up is confusing Boo’s voice with the adult performers who voice Sulley, Mike, or Celia. Gibbs is specifically credited with the child’s original English vocal performance; later language versions use different voice actors.