Invisible Theater is a form of performance in which the show is enacted in a place where people would not normally expect to see a show, for example in the street or in a shopping centre, similar to busking, except without the need for payment. It is generally recognized as having been originally developed by Augusto Boal, as part of his Theater of the Oppressed - which focused on oppression and social issues, during his time in Argentina in the 1970s. Boal went on to develop Forum Theater.
Invisible Theater can give people who would not normally have the chance to see plays the opportunity to do so -- or, as is often the case, it can be performed without the knowledge of the 'audience,' which in such a scenario would consist of whoever happens to wander by. This can be done in order to help actors make a point publicly in much the same motivational vein as graffiti or political demonstration, or it can be done in order to help actors gain a sense of what a realistic reaction might be to a certain scenario; for example, a heated argument over a political or social issue. This type of theatre is performed in public on unexpected bystanders, whom the actors will try to get involved in.
- from Wikipedia