Would you have watched 'Friends' if Courteney Cox had played Rachel instead of Jennifer Aniston? How about if 'Facts of Life' star Nancy McKeon had played Monica? Those stories and more have been revealed in a new history of the hit sitcom.

In the upcoming book 'Top of the Rock: Inside the Rise and Fall of Must See TV,' Warren Littlefield, the former president of NBC Entertainment, recounts the tales of how some the network's biggest shows came into being. The book features "oral histories" gleaned from talking to the stars and creators of those shows, and in an excerpt published in Vanity Fair, the famed lore of 'Friends' is explored.

Co-creators Marta Kauffman and David Crane say not only was McKeon almost cast as Monica, the character itself was initially intended to be far more cynical.

“When we originally wrote the role, we had Janeane Garofalo’s voice in our head,” explains Crane. “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.”

"We originally offered [the role of] Rachel to Courteney Cox,” adds Kauffman, “but she said she wanted to do Monica, not Rachel.”

And the role of Chandler? Matthew Perry auditioned and did well, but was locked into another show at the time, so the search continued. “We brought everybody in," Crane said. "We were so sure that Chandler would be the easiest part to cast. It’s got the most joke jokes. It’s sarcastic and kind of quippy, but no one could do it. No one.”

Craig Bierko was ultimately offered the role, but turned it down. “Thank God! There was something Snidely Whiplash about [him],” said 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.” After Perry was released from his other show, 'Friends' producers snapped him up.

But once the cast was in place, Kauffman knew the ensemble was special: “The first day we went to a run-through, and the six of them were together for the first time, onstage in the coffee shop, I remember the atmosphere being electric. A chill ran down my spine.”