They stay on one level. They often don't build, escalate, or resolve. They often don't reveal anything or challenge anyone. Improvisations must go somewhere. They must improve.
Some people believe that improvisations are field days for the uninhibited.
This belief attracts some wild and insensitive personalities to improv sessions while it repels the more serene and complacent types. An improvisation should never be out of control or physically dangerous for anyone. Blind abandon and bald inhibition have nothing to do with improvisation. Most people think of improvisation as a theatrical exercise. Heading over to the theatre for an eight-o'clock curtain, you can never be quite sure how the next few hours will unfold.