Bet you forgot that once upon a time, Julia Roberts guest starred on Friends. Or maybe you need a reminder of who Rachel's famous sisters are played by. The well-loved show, which is thankfully not leaving Netflix anytime soon, played host to numerous guest stars over its 10-year run, including some big names who went on to earn Emmy Awards for those guest spots. For a refresher on some of our favorite guest stars, keep reading, and then see how well you actually know the show with this tricky quiz. Only true fans of the Central Perk regulars will prevail!