Back when improvisation began picking up speed, there weren't extensive manuals or Google searches. So, improvisers would perform and note what worked, and what didn't work. From their observations, they began to figure out some basic principles of improv.
The following are called "The Rules," or the "Boarding House Rules," or the "Westminster Place Kitchen Rules" which Elaine May and Ted Flicker developed at the St. Louis Compass (a predecessor to Second City).
1. Never Deny Reality. If another actor establishes something as real, the other actors cannot negate it.
2. Take the active choice. Whenever an actor is faced with a decision during a scene or a game, the actor should always choose the one that will lead to more actions.
3. It is the actor's business to justify whatever happens onstage. An actor cannot invent a character that can deny the reality of the scene by claiming "it is out of character." In improvisation, your character is actually you, but with a few additional characteristics.
(Taken from THE FUNNIEST ONE IN THE ROOM, p. 52-53)
Take a moment to let those babies sink in. We'll talk more on each later.