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Pitchfork: Interviews: South Park's Matt Stone

"Cartman is just a great character. You bring a lot of that emotional baggage about him to the song. And frankly, Trey's a fucking great singer and performer himself, and he's really great when he's disguising himself as a character because he's not as vulnerable as a personal singer. And I don't know. We had to hear him do Lady Gaga."

South Park is amazing, including the current season.   This interview cleared some things up for me, especially about which guests stars were real or fake, and where the theme song came from.