"Matthew Perry was one of the first names on our list, but he was doing another show at the time, so we made an offer to another actor, who thankfully turned it down, and we got back to Matthew," she said.
The actor who turned it down was Jon Favreau who later appeared in the sitcom as Pete Becker, a character that dated Monica briefly.
Kauffman also revealed one Friends character was written specifically for the actor who played it.
"David Schwimmer had auditioned (for us) the year before for another pilot, and he was in our heads when we wrote Ross — his voice, the hangdog thing," she said.
Once all the roles were cast, Kauffman said she realised during the first rehearsal the show was going to be something special.
"We had all six of them onstage for the first time, and they read the scene in the coffeehouse," she recalled. "I got chills up and down my spine and thought, 'This is special. There is something about these six, this script for them, that's special'."
Despite ending 15 years ago, Friends is still incredibly popular and is available to stream on Stan in Australia.
"It's a comfort-food show," Kauffman told Rolling Stone about the show's enduring popularity. "These are trying times, and certain people want the comfort food rather than the difficult, mean-spirited kind of show."
Oh, and despite the wishes of fans around the world, Kauffman has ruled out ever getting the Friends gang back together for a reunion episode.
"The show is about a time in your life when your friends are your family," she said. "It's not that time anymore.
"All we'd be doing is putting those six actors back together, but the heart of the show would be gone.
"Two, I don't know what good it does us," Kauffman told Rolling Stone. "The show is doing just fine, people love it. (A reunion) could only disappoint."