Here is what I've done when put on the spot. STOP PLAYING....then ham it up on the microphone. Saying things like "anyone notice something missing"? or "notice how any one can stop playing and the song will still sound alright, but when the bass stops there is a big hole", "Am I right?, come on do you agree?" etc, etc, have fun with it. Get people involved. The band I was in at the time eventually stopped asking. But recently, if asked I just play what I was already playing, just a little louder. Or as someone mentioned before, have some little lick in your back pocket to play, step forward and play it, then step back.