There's no doubt about it that Friends was a success thanks to the comedic chemistry of its central (perk) cast members Courteney Cox, David Schwimmer, Jennifer Aniston, Matthew Perry, Lisa Kudrow and Matt LeBlanc. But, as with any sitcom, it was a total case of Hollywood sliding doors that these six ultimately landed their roles. Each and every one of them had their own potential hang-up that might have prevented them from joining, changing the nature of the series altogether.
In an oral history of the show with Vanity Fair, co-creator Marta Kauffman revealed that she and David Crane originally envisioned the role of Monica going to another actress altogether (and, no, it wasn't Ellen Degeneres as many might believe). "We originally offered [the part of] Rachel to Courteney Cox, but she said she wanted to do Monica, not Rachel. 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." Other actresses also read for the role, but eventually they agreed Cox would be right for the part. Said Crane, "We decided that, week after week, that would be a lovelier place to go."
As Monica might say, ta-ha!