LARP or Live Action Role Playing Games are a form of collective storytelling in which participants physically portray alternative personas or characters. Unlike acting, the story’s outcome is generally not firmly fixed; alternative endings, novel solutions and even failure are possible. LARP is generally a participatory undertaking, and while not every character may be prominent in the story at all times, observers, as is the case with theater and performance, are not typical.
Often, a group of staff or organizers helps establish the basic premise of the story and help guide elements in its development. They present challenges, opportunities and secondary characters (NPCs or Non Player Characters) to name a few examples. They are also the final arbiters of the LARP. You can think of them as referees and facilitators.
Some LARPs involve both creative and mechanical character development, with characters acquiring new abilities, knowledge, motivations and story elements over time. Elements of the story can be resolved by various means including social interaction, creativity, problem solving, conflict and rules of game play.