The old vaudevillans, folks like Red Skelton, Phil Silvers, Bob Hope, and others, could start a skit with one another without practice, live on stage by whispering a set of key words. They could also change the skit in mid-performance by using key words they knew and the audience would think was part of the presentation.
And there were several hundred of these skits...memorized.
My youngest brother was fortunate to tour, right out of college, with a Toby show, a copy of one of the plays was taped and is now in the Smithsonian. In addition to learning how to set up a BIG tent in a high wind, he learned several of the old skits and their cue words. In Minnesota that summer he was lucky enough to "do" one with Red Skelton -- who promptly changed what they were doing in mid-skit!