I learned it in 1964 in a workshop run by The Committee, the satirical performance troupe in San Francisco, as a warm-up exercise for improvisational theater. Spolin’s seminal book Improvisation for the Theater. Even in so simple a game, the role of gamekeeper is important.
In Mirror, the players pair off; if one is left over, the gamekeeper becomes his or her partner. She states the rules, gives advice, and monitors the process.
He moves his body as he wishes, and B tries to be his precise mirror – that is, if A waves his right arm, B waves his left in the same way. In phase III, no one is designated as leader; motion is free, subject only to the condition that the two keep mirroring each other.
Playing mirror with new players, she makes each step clear, and does not tell the end before the beginning.
When they are ready for phase I, she tells them its rules, suggests that they will mirror better if they watch each other’s eyes rather than extremities, and asks A to move slowly enough for B to follow.
When phase I has run for about four minutes, she tells them to trade roles and continue, and watches as they do, reminding them if necessary about eye-contact and slow motion.
When she judges phase II complete – about the same length of time – she asks them to keep their motion continuous and mirroring, but now to handle the problem of leadership as they will, without a designated leader.
She lets phase III run as long as seems useful, usually five to ten minutes, and announces its end.
If there is to be a discussion afterwards, she moderates it.
Here again her approach is not only facilitative, but carefully structured.
For after every game, the first impulse of the players is to babble at random about the experience, yet a more orderly readout and analysis is always more useful.
In general, it pays to rehash a game phase by phase, with the gamekeeper deciding when to proceed to the next phase.