Wikipedia: Improvisational theatre

Improvisational theatre (also known as improv or impro) is a form of theatre in which the actors perform without a script. There are several types of improv, including:

Short form improv, which consists of short, unrelated scenes
Long form improv, in which the scenes are interrelated in such a way as to form a long narrative
Improv games, in which the performers attempt to create a comprehensible scene while conforming to certain specified and possibly restrictive rules.

In all forms of improv, the actors continually construct the dialogue? and action as they perform. Because of the unpredictable nature of such a performance and the unexpected events that occur, improv lends itself naturally to comedy, but it is also possible for improvised scenes to be seriously dramatic.

In order for an improvised scene? to be successful, the actors must accept certain standard principles which allow the scene to advance. With each spoken word or action in the scene, an actor may make an offer, which means that he or she defines some element of reality: this might include giving another character a name, identifying a relationship, location, or using physical gestures to define an invisible space object. These activities are also known as endowment. It is the responsibility of the other actors to accept the offers that their fellow performers make; to not do so is known as blocking, which usually prevents the scene from developing. In violation of this principle, some performers may deliberately block (or otherwise break out of character) for comedic effect (this is known as gagging), but this generally prevents the scene from advancing and is frowned upon by improvisers. Accepting an offer can also be accompanied by adding a new offer, often building on the earlier one; this is a process which improvisational actors refer to as "Yes and..."

The unscripted nature of improv also implies no predetermined knowledge about the prop?s that might be useful in a scene. Improv companies may have at their disposal some number of readily accessible props that can be called upon at a moment's notice, but in general improv performers will use miming techniques to create space objects. As with all other types of improv offers, actors are expected to respect the validity and continuity of space objects defined by themselves and by other performers; this means, for example, taking care not to walk through previously referenced walls or tables.

Because improv actors may be required to play a variety of roles without preparation, they need to be able to construct characters quickly with physical representation, gesture?s, accent?s, voice? changes, or other techniques as demanded by the situation. The character may be of a different age or sex than the actor. Character motivations are an important part of successful improv scenes, and improv actors must therefore attempt to act according to the objectives that they believe their character seeks.

Many improvisational actors also work as scripted actors, and improvisational techniques are often taught in acting classes. Many [theatre troupe]?s are devoted specifically to staging improvisational performances. One of the most successful international franchises is Theatresports?, which was founded by [Keith Johnstone]?, an English director who now lives in Calgary, Alberta. Johnstone wrote what many consider to be the seminal work on improvisational acting, Impro. Other important figures in the development of improvisational theatre were [Viola Spolin]? and her son [Paul Sills]?, founder of Chicago's famed Second City troupe and inventor of [Story Theater]?.