In the 21st season premiere episode co-written by Seth Rogen, Comic Book Guy casts Homer as the lead in “Everyman,” a feature film based on the new comic book superhero he created. To whip him into shape for the part, the movie studio hires celebrity fitness trainer Lyle McCarthy (guest voice Rogen) but Homer has trouble sticking to his new, healthy regimen on the season premiere episode "Homer the Whopper" of "The Simpsons." © 2009 TCFFC ALL RIGHTS RESERVED. ©2009 FOX BROADCASTING. Photo courtesy of Fox.

No arm twisting was necessary to get Seth Rogan onboard The Simpsons.  The Knocked Up star has loved the animated series since its inception.

He has not only helped write the 21st season premiere episode, but he’ll also voice the character, Lyle McCarthy, who is a fitness trainer hired by the studio to get Homer in shape for his starring role in Comic Book Guy’s superhero movie, Everyman.

Click onto media bar to hear Seth Rogen gush about The Simpsons.

Rogen had a whole set of time constraints getting the story done for the show, what with that healthy movie career and all. But he says that his writing partner, Evan Goldberg and he did the best they could, then left the details to the pros - The Simpsons‘ writing staff.

Click onto media bar for Seth Rogen’s experience writing for season premiere episode of The Simpsons airing tonight, 8:00 p.m. ET/PT on Fox.