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The original plan for The One with Rachel's Other Sister (2002), was to bring Reese Witherspoon back as Jill Green. Witherspoon was unavailable, so Christina Applegate was cast as Rachel's (Jennifer Aniston's) other sister, Amy Green. Applegate went on to win an Emmy for the role.
Joey was originally written as a jerk. When Matt LeBlanc questioned why the others would be friends with him, the character was changed.
When Matt LeBlanc auditioned for the role of Joey, he had eleven dollars to his name. When the cast got their first paychecks, the first thing that LeBlanc bought was a hot dinner. Courteney Cox bought a car.
When Courteney Cox dressed as fat Monica for the first time, Matthew Perry walked right past her without recognizing her.
Courteney Cox was originally supposed to play Rachel. In Friends: The Reunion (2021), she explained that she asked to play Monica instead, since she related to her more.
Lisa Kudrow and her best friend wrote "Smelly Cat" one night when they were drunk, and it eventually got written into the show.