This is a model I have used in some game sessions (partially at the end of my Loose Toys campaign) which I think would also make a good drill.
One participant describes an event: if you are using this as the warm up for your RPG session, then the GM should provide the event, but any player should do so. This event can be as subtle or dramatic as the players want it to be.
Going around the group in any order, each player narrates a consequence of the event: they can either state what happens or play out a scene depicting it with the co-operation of other players.
I suggest using a standardised set of 'pauses' between each scene, starting from the event itself. Therefore, the first scene takes place 1 minute after the event; the second takes place 1 hour later; the third is 1 day later; the 4th is 1 week later; the 5th is 1 month later; the 6th is 1 year later; the 7th is 1 decade later; and the 8th is 1 century later. Each scene must expore or relate to the original event or the consequences ascribed to it in a previous scene.
This drill encourages co-operative story telling, stretches the imagination (to milk as much out of the original event as possible) and gets everybody to think about how immediate actions can have long term costs or benefits, which may be of assistance in some types of game.
This sounds a lot like the Ripples in Cardboard People. Only better stated.
I've been in plenty of improv sessions that have ammounted to very little because the participants weren't willing to take what another actor is doing. Everybody wanted to go their own little way. It takes a lot of practice to get people to cooperate on a narrative without any sort of system. This could operate as something like that. I can imagine it being played as a warm up drill for one of the local dramatic societies.
Luck,
- Kirk