The old vaudeville spots were ad libbed, but derived from set peices. Thus, one vaudevillean could work with another if something untoward happened to his or her partner.

"Larry's drunk, so I'm working with you, Sam."

"Fine. Wanna lead off with the ostrich bit?"

"Okay, and follow that with 'Eggs'."

Like musicians planning a gig:

"Let's hit 'em with Maui first, and then Elinor."

Some of the old vaudevilleans had memorized literally hundreds of sketches and could even segue from one to another if the first was falling flat with the audience -- something you can't do with songs. I mean, realizing halfway through it that "John Peel" isn't clicking with the audience and switching to the middle of "John Henry" won't usually work very well.

My brother Ted and I can do this to a small extent:

"Shall we join the ladies?"
"Oh yes! Let's join the ladies and make one BIG lady!"