In Season 3, Episode 1 (“Rainforest Shmainforest”), Jennifer Aniston voices Ms. Stevens, the leader of an environmentalist choir tour looking for recruits at South Park Elementary. Kyle Broflovski (Matt Stone), Eric Cartman (Trey Parker), and the rest of the core four get in trouble for making fun of the group and are forced to join them on their trip to Costa Rica. During the expedition, the party gets lost in the rainforest and gets captured by an indigenous tribe. Ms. Stevens is almost sacrificed wearing a cheerleading outfit, causing her to drastically change her views on the rainforest.
On the DVD commentary for Season 3 of “South Park” (via), Matt Stone and Trey Parker talked about how they were fortunate to work with Aniston and that she was such big fan of the show. He also talked about how despite her being a successful TV star, she seemed quite nervous in the recording booth at first.
“It was just obvious she hadn’t done that kind of acting before,” Parker said. “She just hadn’t. Once she stopped shaking, she did a great job. I think she’s really funny in this.” Aniston’s part as Ms. Stevens is hilarious and is another successful part of her comedic legacy.