Excerpt from USA Today online: (Other parts of interview did not mention South Park...)
Q: How did South Park creators Matt Stone and Trey Parker entice you to be in an episode battling an evil Barbra Streisand?
A: They sent me two episodes with a note asking me to participate.
One of the episodes had George Clooney as a gay dog, so I
thought they probably wanted me to be a gay cat or something.
I thought the show was hilarious, and I was flattered to be
involved. I was surprised when I first read the script and realized
I would be myself in the plot. It was an added bonus because it
impressed the younger members of my extended family. They think
being in South Park is what I do for a living. It was surreal
inhabiting
that world with Cartman. I'm glad I was the hero.