An engaging way of seeding the constraints for the improvised piece.
Preparation
This activity plays with the roles of Audience and Performers as separate. If you don't have a stage, consider adjusting the space accordingly, so there is an area dedicated for performers.
Setup
Assume the roles of ~3 Players and more people serving the role of the make-believe Audience (proportions depending on the number of participants).
Gameplay instructions
Core part of the process is the starting make-believe arrangement when all participants (in their roles as Audience or Performers) discuss the work as if it already happened. Options include reflections, praise, questioning, reveals etc.
Game end
The second and final phase is for Players to perform the work that was so vividly received before.
Editor's notes
Licensed under CC BY-NC-SA 3.0 Générique is an activity intended for use with broader performance field (not only music) and it comes from: http://everybodystoolbox.net (the URL is probably a proper license attribution, although it's not specified clearly). On the page you may find more suggestions, especially for using the activity in the staged context with an actual audience. Here is a (minimally edited) characterization of the activity from its proponents:
Générique is an open-source performance project, which develops depending on the interest and investment of a variety of performers. It is based on a game structure that nurtures make-belief and collective creativity […] Générique means "generic" in French, as well as the credits at the end of a movie. The performance is characterized by those who perform it.
Générique is similar to some activities from improvisational theatre, like "Press Conference" or other games based on endowment. Interestingly, it may be used not only for coming up with a new work, but also to guide interpretation of existing works, it may even serve as an additional constraint when playing other music games.
Click buttons to add all relevant tags:
free-improv on-off (constellations) in-genre algorithm ambient loop minimal
dance draw (art) gestures guess interpret listen
composers events insert narrative quick random simple
dimensions rhythm speeding-up timed tonal
acapella found (+preparation) keyboard
Finish tagging (_game) (regular library entries)
Finish tagging (_non-game) (rare, mostly to hide the above buttons)