Larry Gelbart himself joined the team in 1955, with Caesar’s Hour. He’s supposedly the model for Kenny Franks in the play. His experience had been as a joke writer, and Caesar’s Hour taught him sketch writing, with basic beginning-middle-end story construction. It was a stage in development that led to his first stage play, collaborating on A Funny Thing Happened On the Way to the Forum, and later famous plays, movies and the television series M*A*S*H.

The character of Brian Doyle is based on Michael Stewart, who (like Brian) was Irish, and who would go on to write for the theatre. He was a writer for Caesar’s Hour, where he had the role of the one who actually wrote down what the group had come up with—a role previously filled by Lucille Kallen and Joseph Stein on YSOS.

Another character that appears in at least the TV movie version of Laughter on the 23rd Floor is Harry Prince, based on Dave Caesar, Sid’s brother who appeared on Caesar’s Hour. But he’s not in the play, at least the version I saw produced.

At the center of it all was Sid Caesar. However he was much more the Max Prince character during the run of Caesar’s Hour. YSOS was controlled more by its director, Max Liebman, and Caesar wasn’t even the star at first. But by Caesar’s Hour, he ran the show.

As a creative performer, Sid Caesar was a comic genius. That’s only implied in the play, though the Marlon Brando’s Julius Caesar movie excerpt suggests one of his particular kinds of mayhem. He was also mercurial, with a growing dependence on alcohol and pills, as Max Prince. Caesar later conquered these dependencies.

An aside: the production I saw sort of makes fun of Max Prince’s saxophone playing. In reality, Sid Caesar started out professionally as a saxophone player. In the funniest sketch on the Sid Caesar Collection Vol. 1 DVD is Caesar as a progressive jazzman Progress Hornsby being interviewed on "Ominous" (parody of the interview program Omnibus.) At the end of it Caesar actually plays jazz saxophone. (The Progress Hornsby sketch on YouTube is different and not nearly as good.)