This isn't necessarily a hidden Friends detail, but could you imagine if the cast had been made up of different actors? Friends could have had a totally different vibe and voice. For instance, Courteney Cox was originally asked to play Rachel.
Executive producer and co-creator Marta Kauffman revealed to Vanity Fair, "We originally offered Rachel to Courteney Cox, but she said she wanted to do Monica, not Rachel." Kauffman and fellow writer, David Crane, initially had a different vision for Monica. "When we originally wrote the role, we had Janeane Garofalo's voice in our head. Darker and edgier and snarkier, and Courteney brought a whole bunch of other colors to it. We decided that, week after week, that would be a lovelier place to go to."
Former head of casting at NBC, Lori Openden, also told Vanity Fair that Cox did, in fact, have competition for the role: "Nancy McKeon, from The Facts of Life, also read for Courteney's part. She gave a terrific performance. Warren let Marta and David make the call. They went off for a walk and came back and said Courteney."
And what of the other actors? Bright revealed to an online question on USA Today, "We held auditions where we saw about 500 actors between New York and Los Angeles. David Schwimmer we worked with before, and the writers had him in mind for the part of Ross. Lisa and Jennifer and Matthew and Matt we cast from auditions."
"It was a fascinating casting experience," Kauffman told Vanity Fair about casting the pilot. "We saw a countless number of actors. One of the first actors on our list was Matthew Perry to play Chandler, but he was doing a show called LAX 2194, so he wasn't available. We brought other people in." They nearly hired Craig Bierko — coincidentally, they later discovered that Perry had coached him. It was actually Bierko who passed on the role after reading the script. "Thank God! There was something Snidely Whiplash about Craig Bierko," says Littlefield. "He seemed to have a lot of anger underneath, more of a guy you love to hate. The attractive leading man who you love and can do comedy is very rare." But eventually Matthew Perry was released from his Fox pilot and the rest is history!
Many people say Ellen Degeneres was offered the role of Phoebe but turned it down. DeGeneres cleared up this rumor in 2015 during an interview with Howard Stern — she explained that her show Ellen premiered around the same time as Friends — except that her show was actually called These Friends of Mine for the first season, so the similar names caused the confusion.