Improvisational theatre (also known as improv or impro) is a form of theater in which the improvisational actors/improvisers use improvisational acting techniques to perform spontaneously. Improvisers typically use audience suggestions to guide the performance as they create dialogue, setting, and plot extemporaneously. Improvisational theatre performances tend to be comedic, although some forms, including Playback Theatre and Theatre of the Oppressed, are not necessarily intended to be comedic.

Many improvisational actors/ improvisers also work as scripted actors, and "improv" techniques are often taught in standard acting classes. The basic skills of listening, clarity, confidence, and performing instinctively and spontaneously are considered important skills for actors to develop.

Origins

Improvised performance is as old as performance itself. From the 16th to the 18th centuries, Commedia dell'arte performers improvised based on a broad outline in the streets of Italy and in the 1890s theatrical theorists and directors such as Konstantin Stanislavski and Jacques Copeau, founders of two major streams of acting theory, both heavily utilised improvisation in acting training and rehearsal. 

Spolin can probably be considered the American Grandmother of Improv. She influenced the first generation of Improv at The Compass Players in Chicago, which led to The Second City. Her son, Paul Sills, along with David Shepherd, started The Compass Players and The Second City. They were among the first organized troupes in Chicago, Illinois and from their success, the modern Chicago improvisational comedy movement was spawned.

When The Second City opened its doors on December 16, 1959, Viola Spolin began training new improvisers through a series of classes and exercises which became the cornerstone of modern improv training. By the mid 1960s, Viola Spolin's classes were handed over to her protégé, Jo Forsberg, who further developed Spolin's methods into a one-year course, which eventually became The Players Workshop, the first official school of improvisation in the USA. During this time Jo Forsberg trained many of the performers who went on to star on The Second City stage.

Many of the original cast of Saturday Night Live came from The Second City and the franchise has produced such comedy stars as Mike Myers, Tina Fey, Bob Odenkirk, Amy Sedaris,Stephen Colbert, Eugene Levy, Steve Carell, Chris Farley, Dan Aykroyd and John Belushi.

Many directors have made use of improvisation in the creation of both main-stream and experimental films. Many silent filmmakers such as Charlie Chaplin and Buster Keaton used improvisation in the making of their films, developing their gags while filming and altering the plot to fit. The Marx Brothers were notorious for deviating from the script they were given, theirad libs often becoming part of the standard routine and making their way into their films. Many people, however, make a distinction between ad libbing and improvising.

The British director Mike Leigh makes extensive use of improvisation in the creation of his films, including improvising important moments in the characters lives that will not even appear in the film. This Is Spinal Tap and other mockumentary films of director Christopher Guest are created with a mix of scripted and unscripted material and Blue in the Face is a 1995 comedy directed by Wayne Wang and Paul Auster created in part by the improvisations filmed during the production of their movie Smoke.

Improv comedy techniques have also been used in television and stand-up comedy, in hit shows such as the recent HBO television show Curb Your Enthusiasm created by Larry David, the UK Channel 4 and ABC television series Whose Line Is It Anyway (and its spinoff Drew Carey's Green Screen Show), Nick Cannon's improv comedy show Wild 'N Out, and Thank God You're Here. In Canada, the long-running series Train 48 was improvised from scripts which contained a minimal outline of each scene. The American show Reno 911! also contained improvised dialogue based on a plot outline.

Improv as a tool for corporate entertainment and corporate training

Major companies such as The Second City and dozens of business training companies use improv exercises to design new and interesting corporate 

Many important elements of successful business development, such as trust, active listening, innovation and the understanding and building of relationships can be developed using exercises created by Viola Spolin, Paul Sills, Del Close, and Keith Johnstone and others.

Participants in improv corporate workshops learn a new way of thinking, a new respect for others in negotiations or transactional relationships, how to be “in the moment“, how to make positive choices that keep doors open, proactive listening as opposed to passive or even responsive listening, trusting one another to do the right thing and the benefits from watching the other guy’s back.

The Process

Improvisational theatre allows an

interactive relationship with the audience. Improv groups frequently solicit suggestions from the audience as a source of inspiration, a way of getting the audience involved, and as a means of proving that the performance is not scripted. That charge is sometimes aimed at the masters of the art, whose performances can seem so detailed that viewers may suspect the scenes were planned.

In order for an improvised scene to be successful, the improvisers involved must work together responsively to define the parameters and action of the scene, in a process of co-creation. With each spoken word or action in the scene, an improviser makes an offer, meaning that he or she defines some element of the reality of the scene. This might include giving another character a name, identifying a relationship, location, or using mime to define the physical environment. These activities are also known as endowment. It is the responsibility of the other improvisers to accept the offers that their fellow performers make; to not do so is known as blocking, negation, or denial, which usually prevents the scene from developing. 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 many improvisers. Accepting an offer is usually accompanied by adding a new offer, often building on the earlier one; this is a process improvisers refer to as "Yes, And..." and is considered the cornerstone of improvisational technique. Every new piece of information added helps the improvisers to refine their characters and progress the action of the scene.

Because improvisers may be required to play a variety of roles without preparation, they need to be able to construct characters quickly with physicality, gestures, accents, voicechanges, or other techniques as demanded by the situation. The improviser may be called upon to play a character of a different age or sex. Character motivations are an important part of successful improv scenes, and improvisers must therefore attempt to act according to the objectives that they believe their character seeks.