Matt LeBlanc's Joey Tribbiani -- how you doin'? -- is one of TV's all-time funniest, most lovable dumb guys, but Joey wasn't originally written that way.

"We didn’t know how funny Matt could be playing stupid," "Friends" co-creator Marta Kauffman says in a new interview for the "Friends" complete series Blu-ray box set.

"That was not written into the part originally," another series co-creator, David Crane, says in the box set clip above.

Adds Kauffman, "And what a big heart Matt has. And he wears it on his sleeve, and it made the character so much sweeter and so much better than we had started with."

LeBlanc went on, of course, to become one-sixth of one of primetime's greatest ensemble casts, on the series TV Guide ranked number 21 on its list of the 50 greatest TV shows of all time. But he, and most of his "Friends" castmates, were not shoo-ins for their roles.

In "Top of the Rock: Inside the Rise and Fall of Must See TV," former NBC entertainment president Warren Littlefield's fantastic oral history of the network's must-see TV years, LeBlanc recalls how he auditioned for the role alongside a guy dressed like a cowboy.v

"I looked at him and thought, 'One of us is way off the mark. God, I hope it's you,'" LeBlanc says in the book.

"Top of the Rock" also reveals other "Friends" casting could-have-beens, including "Facts of Life" star Nancy McKeon, who was Courteney Cox's main competition for the role of Monica; comedy actor Craig Bierko, who passed on the role of Chandler; "Will & Grace" star Eric McCormack, who auditioned for the role of Ross; and "Square Pegs" star Jami Gertz, who was offered the part of Rachel and passed.

Littlefield considered Gertz's refusal of the role a boon for the network.

"One night while gassing up my car on Sunset Boulevard in Hollywood, I ran into Jennifer (Aniston), and she asked me, 'Will it ever happen for me?' God, I wanted it to," he writes in his book. "I didn't care what it would take -- this was the role for her."

The rest is TV history, and part of the "Friends: The Complete Series Collection" Blu-ray box set, which will be released on Warner Bros. Home Entertainment on Nov. 13.