Taye Diggs' first show in HEDWIG...he tripped over the microphone wire, slid/fell on the car, and messed up a punchline of a joke...his improv was really great with all those little things.
And, of course, John Cameron Mitchel is an absolute master of improv. First thing that comes to mind is when a few people came in after the opening song, sat in the front, he took their drinks, drank them, and dumped the remainder on the latecomers' head. What a show.