Truth in Comedy: The Manual of Improvisation by Charna Halpern, Del Close, Kim Johnson

when the routine is disrupted that the action of the scene begins.
    And what results is usually far more interesting than what was planned.
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KEEPING AN OPEN MIND
    There is a big difference between a strong, information- filled initiation that makes assumptions, and a preconceived notion used to control a scene.
    For example, the opening exchange in the scene between the President and the Doctor starts out with an opening line and an assumption, but the player (presumably) isn't trying to promote a pre-planned scenario. If he was, the equally presumptuous response probably demolished any intended plot. The biggest mistake the first player could make would be to downplay his partner's response in order to continue shoving his scenario down the throat of his partner.
    Having an idea is not bad in itself, especially if the actor conveys it easily to his partner through a simple initiation, such as a line of dialog or a physical movement. The simpler the idea, the better.
    It is vitally important, however, for an improviser to drop his idea immediately the moment the scene takes an unexpected twist. Of course, it doesn't make much sense for one player to devise an elaborate plot for the scene.
    When all the players are involved in its creation, the scene is much more interesting. Two heads are better than one, and in Harold, six or eight heads are even better.
    A common mistake for some improvisers is to be led by the audience. If the crowd laughs loudly at one particular moment, the performer may be tempted to push the scene in the direction that the audience is responding to — instead of responding to his fellow performers.
    Unfortunately, an_audience doesn't necessarily want what it thinks it wants. A player is usually much better off listening to his fellow performers and director than the audience members. George Wendt remembers that during his days at Second City, it wasn't enough to make the crowd laugh.
    "Del said, 'We don't care if it works for the audience — it has to work for us,'" says Wendt. "At that time, an improv scene that we may have become fond of because it got a lot of laughs had to work for Del, (producer) Bernie Sahlins, and (pianist) Fred Kaz — all three of them — or else it would not be considered for our Second City show. 'Don't tell us it works — we'll tell you if it works.' "
    START IN THE MIDDLE
    Exposition sucks.
    Backstories and explanations are rarely the most exciting part of any book or film; generally they are a necessary evil.
    In improvisation, actors are seldom hamstrung by exposition. Instead, they simply ignore it all, and begin their scenes in the middle!
    Nothing is more boring or wastes more time than two improvisers starting a scene with "Who are you?" It is always helpful if the players know each other (or their roles) when they begin their scene; they need to make assumptions about their relationship right from the start.
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When two players pretend their scene actually began five minutes before the lights went up, they make discoveries much more quickly. They spare the audience their excruciatingly dull groping around for information that should simply be assumed.
    SHOW, DON’T TELL
    An improviser accepts what his partner says as a gift, and builds on that idea. He may respond with another gift, and the two of them build their scene based on the information in their statements.
    They must make active choices, rather than passive ones, and then follow through on their ideas. Everything said can be heard and used, even what might be considered a mistake. Since "action begins with the disruption of a routine," the "mistake" could be the disruption that begins the action.
    Too many actors make the error of talking about doing something instead of doing it; a potentially interesting scene gets frittered away because no one is actually doing anything. If the idea is active, it leads, step by step, to the next idea. But if the idea is talked away, the actors never arrive at