Could you imagine if the cast had been made up of different actors? Executive producer and co-creator Marta Kauffman revealed to Vanity Fair that they originally offered the role of Rachel to Courtney Cox, but she asked to play Monica instead. Kauffman and her writing partner, David Crane, initially had a different vision for Monica. "We had Janeane Garofalo's voice in our head. Darker and edgier and snarkier," but Cox brought dimension to the role that they hadn't even imagined. Former head of casting at NBC, Lori Openden, also told Vanity Fair that Cox had competition for the role: "Nancy McKeon, from The Facts of Life, also read for Courteney's part. She gave a terrific performance." As we now know, not as good as Cox.
And what of the other actors? Executive producer Kevin Bright revealed to USA Today that they auditioned 500 actors for the show. He also revealed that they knew David Schwimmer and had him in mind for Ross, while "Lisa and Jennifer and Matthew and Matt we cast from auditions." Kauffman told Vanity Fair that Matthew Perry was one of the first actors they wanted to hire, but he was already doing another pilot. They nearly hired Craig Bierko, but he passed on the role after reading the script. "Thank God! There was something Snidely Whiplash about [him]," said Warren Littlefield, the former president of NBC Entertainment. Luckily, Matthew Perry became available, and the rest is history.
In a 2015 during an interview with Howard Stern, Ellen DeGeneres cleared up the rumor that she was asked to play Phoebe — she explained that her sitcom Ellen premiered around the same time as Friends — except that her show was initially called These Friends of Mine for the first season, so the similar names caused the confusion.