I've seen a few fights happen at gigs or clubs and without even thinking have jumped in or started to before it was handled by bouncers.
Years ago I was with friends and one of the guys at the table punched his girlfriend in the face and was about to do it again when we all jumped on him and knocked him to the ground when the bouncers appeared they gave him a good thumping too and then kicked us all out and banned the couple from ever entering again. You react or I do without thinking for a moment that you might get hurt too.
I like to think I'm not such a risk taker now, about 2 months ago from the tobaco shop I wasa working in, across the street were a bunch of parked scooters and motorbikes, a drunk guy comes along walking and fell into one and the whole bunch went down like a set of dominos. Unluckily for him one of the motorcycle guys saw it and jumped him and started kicking him in the head while he was down. We called the police from the shop and about five minutes later they arrived and instead of getting the guy who'd given the beating, they came in and bought cigs!