A loose term referring to a certain very odd subset of
improvisational theater. Guerrilla improv is
unscheduled, usually not
scripted, usually only barely thought out, and most importantly at all:
the audience almost always has no indication at all that what they are looking at is a theatrical performance. The audience is everyone, the universe, whoever is in the area at the time,
innocent bystanders who are unaware they are an audience; they simply are walking down the street or through a store and see some kind of
disturbance, witness some scene unfolding before them, and walk off thinking they have witnessed a small
glimpse through a briefly
opened random window into someone's
private life they were not meant to see.
What they have actually seen was a manufactured scene.
And the audience is usually involved, in some way drawn in, in some way made to be participators and not just passive lurkers.
Good examples would be randomly performing spy-noir-esque melodramas outside restaurants, with the single intent of designing your setup such that everyone involved believes they actually saw something really happen.
I know a number of people who have an interesting ritual they perform randomly at high school theater conferences; they will go up to a random victim picked out of the crowd, read their nametag (everyone has a nametag; lets say this one says "Fred") and tackle-hug them as if they were incredibly happy to see them, and say something like "Oh my God!!! Fred!!! I thought you were dead!!!"
"What?" says Fred.
"You don't remember me?" says the performer. "Steph? I was in your class in the fifth grade!"
From there the performer attempts to convince Fred, without letting Fred know how he is being worked on in any way, that they used to know each other really well. If done right, eventually memory will map blurs to reality, and the audience will believe. Somehow this almost always works.
Like guerrilla warfare, guerrilla improve strikes out of nowhere and seems senseless and chaotic no matter how complexly intricate it may be under the surface.
This is not necessarily done for comedic purposes; if you can't understand why else it would be done already, well, i can't really explain it, and if you can't come understand the object by yourself you never will.