For physical battles like sword fights, I visualize the fight and choreograph each character's moves. I often get up from the computer and pretend I'm holding a sword, imagine the opponent's move, and block it noting my balance, what I'm leaving open, and possible return blow.
I also use the physical location of the hero to vary fighting. The floor may be bloody from his first opponent so the hero or villain may slip and fail to parry a blow, etc.
I rarely write out blow for blow because I think that's boring. Instead, I'll give occasional overviews of what's happening. For example, the hero is thinking about how his body is learning the rhythm of the fight, or he's aware of other fighters around him.
I try to avoid using technical terms to describe the fight because I'm writing as much for those unfamiliar with swordplay as those who are, but I try to be accurate about how to use the weapon, and I use a sprinkling of correct terminology to make it seem more realistic.
I've never fought with a sword, but I've held a number in my hand, and I've watched others fight with them. I try to remember the weight of the weapon, the sound a fighter makes as he swings the heavy sword, and the sheer weariness of the weight of fighting something or someone above you.
This advice holds true in any other form of action scene, too.
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