From George Clooney to Julia Roberts: The Best Guest Stars on Friends

It may be all about the core six, but the gang had many famous faces join them along the way

SEAN PENN

In the season 8 episode "The One with the Halloween Party," we met Eric, a sweet school teacher who is engaged to Phoebe's twin Ursula, thinking that she's a similarly sweet teacher. After Phoebe breaks the news, they date for a bit – until he realizes he can't stand the sight of Ursula, and therefore, Phoebe too.