The Play's the Thing is designed as a one session game of directed improvisation and roleplaying, lasting two to three hours. It must have five, and only five, players: at least two men and two women. If there are three female rather than male players, the character of Gregory can be renamed to Katrina; no other changes to the game need be made.
While there may only be five players, I believe the game would be enhanced by an audience for the play.
The game has two major sections. It begins with the characters discussing the production of the play - most importantly, who is going to take which part, but also any other matters that are of interest. The more thought that goes into the play before-hand, the better the play will be.
The second section is the performance of the play itself, and the most dramatic end for the whole session is at the close of the Monarch's lament in Act III, Scene 3.
The GM's biggest part to play in this game is informing the characters of the "real" outcomes of actions taken within the play, after David and Gregory have performed the ritual. For the ritual does indeed work, with consequences that not even Gregory has foreseen.
Any injury inflicted on a role in the course of the play is really suffered by the character, and that character should be informed of this when they come off-stage.
If a role dies, then the character is also dead. This means that during a scene-change, the dead body must be dragged off-stage. If the character is playing a second role who has a later scene, the dead character comes to life for as long as that scene lasts (and can make it off-stage), before falling dead again.
Emotions fall under the sway of the ritual's magic, so that whatever a role feels, the character also feels. This is important for off-stage moments, and in particular for the Monarch's final lament.
When the characters begin to realise that something is wrong (as when a character dies, for example), they may try to stop the play or refuse to continue. Such attempts make no difference - the show goes on, with them participating, however unwillingly. Allow the characters to be somewhat reluctant in speaking their lines, but don't let them overdo it. This is potent magic.
Whether the understanding of the characters as to what is going on makes any difference to the potency of the magic on them or not is up to the individual GM. I favour it not having any effect at all, so as not to spoil the high death rate, but others may wish for a slightly less bleak ending. Regardless, the demon (see below) is outside this magic (though it was summoned with it) and can take characters' souls to Hell even if they know everything.
In Act III, Scene 3, when the Heir is supposed to bargain with the Demon who is empty air, it is time for the GM to enter stage left, as the Demon. At this point, the magic of the ritual is partially broken - the Demon should let the character playing the Heir know that s/he doesn't have to follow the script at this point. S/he is free to choose whether the character playing the Noble gets taken down to Hell, or whether s/he will make the sacrifice willingly. This is, really, the crucial scene in the game, as well as the play - with any luck the player has been lured into a position where a natural choice will follow.