Seth Rogen has fulfilled a dream, and he's not talking about starring in the box-office hit Knocked Up or playing a superhero in the upcoming The Green
Hornet.

Rogen co-wrote an episode of The Simpsons and lends his voice to a character in the episode that airs on US TV tomorrow, kicking off the Fox show's 21st season.

He's only the second visiting celebrity to both write and act, following in the 2006 footsteps of Ricky Gervais.

"As a writer, it always just seemed like the Holy Grail," Rogen said of the animated series. "I can die a happy man now."

The episode, by Rogen and writing partner Evan Goldberg in collaboration with the show's writers, plays off the actor's pre-filming fitness regimen for Green Hornet. The movie is in production and set for release in 2010.

In Homer the Whopper, Rogen plays a trainer assigned to get Homer Simpson in superhero shape to play Everyman, a hero created by Comic Book Guy and played by the rotund couch potato.

"We wanted to comment on how Hollywood generally ruins these movies. The whole joke is Homer is cast to play a guy who's an everyman and they try to make him into this physically fit guy," Rogen said.

Recording with the cast was "one of the highlights of my life", said the seriously devoted Simpsons fan (favourite episodes include Itchy & Scratchy Land and the one in which Bart sells his soul to Milhouse).

"It was completely surreal. I was just in shock afterward. I felt like I had gone skydiving or survived an earthquake," Rogen said.

Other celebrities to be heard on The Simpsons this season include comedienne Sarah Silverman, actress Angela Bassett, American footballer Eli Manning and sportscaster Bob Costas.