Anytime during this scene the players can be interrupted by you [audience]. If at anytime you don’t like what they have said you make them change their sentence by yelling, ‘should have said.’ It is important to give the players a chance to build a story, but if you don’t like the way that it is going then yell, ‘should have said.’
Build a scene like any other. Give a bit of extra pause between your comments to allow for the audience to interject. Listen closely for the audience’s input. They will hear the ‘should have said.’ If you miss one expect to be eaten alive. When you are expected to change what you said make a strong choice that alters the course of the scene. They have yelled it because they don’t like what is happening, so change the way the scene is going.
Replace long drawn out comments with short ones.
The callers can be limited to one or two audience members is chaos is imminent.