Self-Judgment. Save that for later, when you're off the stage.
Most improvisers who have stuck with it are intelligent, and being intelligent people they can often be highly critical of themselves and others. They have a tendency to examine what they're doing, and this is a good thing. Aristotle said, “the unexamined life is not worth living.” Good point Aristotle.
But, an improviser will also note, “the examined life needs to wait until you’re off the stage.”
Nothing is worse than (forgive the cliche) analysis paralysis--the tendency to choke under pressure because your mind is going one million miles an hour. I'm no good, I'm not going to be funny, Dad was right--I should have died instead of my older brother, etc. (you don't know what crazy thoughts will come out when the self-doubt monster strikes).
So, how do you turn off those critical voices in your head and perform to the best of your abilities?
Cultivate an attitude of "there's nothing to lose." The truth is, you're going to mess up sometimes. You're going to do scenes that nobody likes (not even your mother). You're going to occasionally choke, or wimp, or waffle, or go big and nobody laughs. The good news is, you're also likely to have moments of brilliance and exceptional comedy if you stay loose.
The best performers/players are either a) so confident in their abilities that they don't think it's possible to lose (that can cause problems on a team), or b) consciously put on the "there's nothing to lose" attitude to suit their needs, when appropriate.
I found this quote in a biography of Del Close. He cultivated the "there's nothing to lose" attitude whenever he went onstage (and, if you read about his crazy life, he lived that way offstage too):
“Just before going onstage, he (Del) would say ‘f$%^ it,’ and then maintain that attitude the entire time he was onstage. Self-judgment and self-criticism were not allowed until after he was offstage (p. 142, The Funniest One in the Room by Kim “Howard Johnson).
This is good advice. Say whatever you need to convince yourself that what you're about to do doesn't really matter --and, paradoxically you'll free yourself up to produce something that maybe does.