A couple weeks ago, I was re-writing an encounter scene between two characters. In the original version of the scene, the encounter held an element of surprise, and thus ended with a panic and a fight. However, in the new version, most of that surprise was gone. As a result, the argument that progressed wasn't so much an argument as it was a negotiation between two cunning and rational characters. As a result, these characters came up with a mutually beneficial solution that allowed them to avoid a fight.
In real life, that would have been the ideal course of action--smooth, rational, nonviolent. But that "ideal course of action" wouldn't have advanced my plot. I needed that fight.
So I went back to the original scene--the one with the element of surprise. Why? Because the fight wouldn't have happened otherwise. Like I said earlier, these two characters are usually not hot-blooded and impulsive. Both of them fight more successfully with words and deceit than with fists and weapons. In order to make these characters descend into violence, I had to put them in the right situation. I had to take away their sense of control and drop them into desperate situations.
This is why it's important to understand your characters--fights are good, but only when they're believable. Sure, some people are naturally hot-blooded and will fight at the slightest provocation, but if your character isn't one of those people, then there needs to be sufficient justification for his or her sudden change of behavior. It doesn't matter how exciting that fight is if your readers find it completely implausible. Remember, you want your readers to be emotionally in-tune with your characters--and if your readers don't relate to that feeling of anger, panic, and/or desperation, then they're going to watch that fight thinking "Wow, why is this character such a fucking moron?"
So that's your writing PSA for the day (Er, week? Month? It's been a while since my last post...) Make your fights believable.