Impro theatre is a technique drama teacher Keith Johnstone invented to teach his students to be more spontaneous. Impro turned out to be entertaining in it's own right though, and a new form of theatre was born. Most times it is done for comedic value, sometimes hysterical comedic, bu there is also a move towards more serious impro, such as Kenn Adam's book "How to improvise a full length play" and Johnstone's own stance on the matter.
Typically impro is done as Theatresports - Two teams face off in a good-natured and playful contest, taking turns of challenging each other to short scenes with given rules and themes. (Could be anything really, for example best enactment of a folk tale, best single actor scene, best scene with a audience member participating)
My experience is mainly from the local group gbgimpro (Gothenburg impro) and their after work impro nights. The formula here being:
1) The group explains the rules for the next scenes and then play it out (For an instance, whenever the audience shouts "Sounds like a song!", the actors has to do a music number of the line sie just said. Another example of rules would be that every actor has to speaks in three word lines)
2) The group asks for volunteers from the audience to play a scene according to the same rules
3) The audience provides a setting/mission for the scene (For an instance; robbing a bank or redecorating a house)
Costume and props are never used, except for chairs, whereas Johnstone is a strong proponent of using sets and props. This would probably clash with the audience participation of gbgimpro, though.
Kenn Adams sums impro up in the three following guidelines:
1) Be spontaneous
Don't plan in advance, act on your partner actors! In Johnstone's books, exercise upon exercise is spent upon training this very simple idea, to react in a second rather than trying to think up something "good".
2) Always make your partner look good
Stressing the collaborative aspect of impro. Beginner impro actors are often so caught up in their own performance and planning (Which they aren't supposed to be doing anyway) that they don't take notice of what the other actors are doing.
3) Always say yes
Don't block the story, advance it! There seems to be a great fear among the actors of impro of actually progressing the story, letting other actors take control, or even to accept their own spontaneous ideas, a sort of achievement-anxiety self-censoring.
Keith Johnstone doesn't provide distilled guidelines in his books, but if you boil it down, these two could be added to the guide to impro:
4) Don't try to be funny, just act on the moment
Going for the quick laughs will only feel forced and won't let the scene develop. In his exercises Johnstone even has his student trying to be as bland as they can, and to stay within the circle of what the audience expects (Which is rather complex, actually)
5) Allow yourself be altered
Johnstone describes how actors fear "losing", having the other actors get the drop on you. For instance, he describes a horror scene the victim takes delight in, instead of becoming horrified. The actor are striving to maintaining control for their roles instead of vanquishing it.
These guidelines captures the very essense of "letting something grow".
Impro is about being in the moment, being one with the scene together with the other actors - To let the scene and the other actors into yourself, so to speak.
Don't plan, listen instead.
Don't act like an actor would act, instead react, lend your body and mind out to the scene and just act on what happens around you, be natural.
Don't stop any ideas or events, let them come out, nourish them, and theatre will grow.