History Photographed on Instagram: "The adorable and authentic voice of Boo was provided by Mary Gibbs, who was only a toddler when Monsters, Inc. was being recorded.⁠ ⁠ Because she was too young to read a script or sit still in the recording booth, the crew creatively captured her dialogue by following her around the studio with a microphone and compiling her spontaneous words and sounds as she played."