The most important rule of improvisation is: accept offers of players. Rejecting automatically stops, disrupts, or impedes every improvisation. Suppose player A says to player B: "Where are the drugs?" B has to respond to the "drugs" in this case, but may not do so as if they didn't exist, for example: "What drugs? There are none!" This is Blocking.
One must therefore carry out the idea of the players, and without taking his own into consideration, or not consistently trying to push through his own ideas.