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In the first couple of episodes, Chandler's (Matthew Perry's) and Joey's (Matt LeBlanc's) apartment number was #4, and Monica's (Courteney Cox's) and Rachel's (Jennifer Aniston's) apartment number was #5. However, in the later episodes, they are 19 and 20, respectively, because the producers noted that 4 and 5 corresponded to apartments on lower floors, while the two apartments were supposed to be higher up the building.
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.