Here's The Story Behind Every Celebrity Cameo In Friends

Billy Crystal and Robin Williams starred in it by accident.

OK, so you think you're a Friends fan.

You know exactly how many times Ross forgot his own birthday, noticed when the wooden rafter disappeared from Monica's apartment, can reel off exactly which cast member was paid the least (before David Schwimmer negotiated them $1m per episode as a group), AND you're aware that Chandler, aka Matthew Perry, dated Susie Underpants, aka JULIA ROBERTS, IRL. 

But do you know how Robin Williams came to be in the show? How they snared a huge star like Bruce Willis? Why Isabella Rossellini's cameo was SO PERFECT? Read on, people - here's the story behind all our favourite Friends celeb cameos.