Summary
4 players stand at the 4 points of a diamond (like a small baseball diamond), “home” being closest to the audience (or the remaining students who are observing) “2nd base” being farthest away.
Details
The player who will play the character, and do a character monologue, is at “home”. The player on 1st base demonstrates a pose or posture - the way the character will stand and move. The player at home takes on that posture - the player on 1st can correct him, if he’s missed any details of the posture.
The player on second demonstrates a voice - how the character (at “home”) speaks. Is there an accent? Is it loud or soft, is it raspy? etc. The player on home tries to do that voice. The player on second can correct for any details.
The player on third demonstrates a physical gesture and catch phrase that the character (on “home”) will pepper into the monologue. (Slapping the thigh and saying “Wakity doo” etc.).
Once the character player at “home” has gotten the posture voice, gesture, and catch phrase someone from the rest of the class gives a suggestion, and the player at “home” delivers a monologue, inspired by the suggestion, as a character with all those characteristics.
Once that’s done, the player at home takes a seat, everyone else “advances a base”, and someone from the class who wasn’t up, takes over first base, and the exercise repeats until everyone has had a turn.