Shearer is crucial to what remains of the show’s legendary dynamic, because of the sheer volume of what he does. Yes, Yeardley Smith, who voices Lisa, is a crucial component to the show, but Lisa is the only character that Smith performs. Julie Kavner voices Marge and her sisters, but Kavner’s vocals are limited exclusively to the Bouvier family. But Shearer (along with Hank Azaria) voices nearly the entire town — scores of characters over the past 26 years. Mr. Burns. Smithers. Principal Skinner. Dr. Hibbert. Kent Brockman. Ned Flanders. And many, many more. And because he’s been with the show since its beginnings, Shearer is more than the voice of these characters; he’s the creator of these characters, and he’s done a wonderful job of cultivating them, from cartoons to fleshed out protagonists.
Source: salon.com