You don't learn to fight by watching Jean Claude Van Damme movies. You can't even learn by watching real fights. To figure out how you handle physical confrontation, and the physics of brawls, you actually have get into one.

There are a lot of guys out there who talk tough, who have confidence in their abilities because they can bench 200 or because they participated in a bar-brawl back in '01 and kicked some drunk's ass. The truth is none of that matters. Each fight exists in a universe of its own, with rules that get made up when the fight starts. There are some general rules, but every setting is different, and every opponent is different. These are a couple of things that the out-of-practice amateur should remember when the time comes. You probably won’t, so this is more for minor entertainment purposes than anything else.

It doesn't usually work the way it did in your head
If you train, hit a speed bag regularly and push yourself, and the guy you are going up against is terrified and also slow and old and fat, then maybe, on the 3rd or 4th attempt you might be able to land that perfect uppercut, or a balletic roundhouse kick. The odds still aren't in your favor though. Your hardest blows miss, when they connect they might hurt you worse than they do him. People don't usually just stand still and let you deliver accurate, on-target punches. A lot of the human head (the usual target of punches) is made of bone. If they are genuinely pissed or scared, or scared AND pissed you will mostly find out that adrenaline can do wonders for the  pain-threshold.

Pain
Brawls aren't usually one-sided. Watch a boxing match. Note how if it lasts past 3 rounds even the guy who is clearly winning the fight has swollen eyes and looks exhausted. Remember, that's with breaks and padded gloves and (usually) very fit athletes. An all-out brawl in which people are more-or-less sober and genuinely enraged, will last maybe 45 seconds at best, for most people, and there will probably be pain on both sides. If nobody grabs a baseball bat there will likely not be a knockout. It's mostly about stamina, who can duck more, or tie the other guy up till he gets tired, and, yes, who can handle pain the best.

Consequences
At some point during the fight you may hear sirens, or your boss/principal telling you to break it up. In the frenzy of white-hot anger people lose sight of the big picture. The people who can lose sight of it the best, who can shut out all the possible bad effects of what is happening now, those tend to win. They also tend to get fired and/or arrested.