Pierce is known for his distinctive voice and, like his Frasier co-star, Kelsey Grammer, is often called upon to provide voice work. His notable roles include the narrator of the movie The Mating Habits of the Earthbound Human in 1999, walking stick insect Slim in A Bug's Life, Doctor Delbert Doppler in Disney's film Treasure Planet, and the amphibian Abe Sapien in Hellboy. Pierce refused credit for his Hellboy role because he felt it was the performance of Doug Jones, and not his own voice, which ultimately brought the character of Abe Sapien to life. He was the voice for Drix, a cold pill, in the animated comedy Osmosis Jones.