When I was in college, I went to see Robin Williams in concert, and he was hilarious (and just as obscene as Richard Pryor or Eddie Murphy, though he did not really have that reputation publicly).
That is not the story. The story is in the fact I went to see him a second night in a row. This seems a dumb thing to do, to go to the same show twice in two nights, but I was chasing after this girl and she wanted to go. At the time, for the right girl, I would probably have gone to a 3-hour Uruguayan poetry reading.
Anyway, the amazing part was... it was not the same show. Yes, the basic structure was there, but huge masses were different. That is when I realized that he was just making it the hell up as he went along, and he was hilarious doing it. I had known intellectually that he had a reputation for improvising way off his scripts, but to actually see it in real time was amazing.