One of the challenges of directing a sword fight is that modern audiences simply don’t perceive swords as weapons; they think of them as costume pieces or props. Too often, when the villain threatens our hero with a sword, no one fears for the hero—they’re excited for the upcoming sword fight. They’re thinking in terms of action, not violence. This is bad.
I was fortunate to visit the home of a gentleman with a massive collection of real swords—so many that he stores them in map cases, six or eight to a drawer—and handle a genuine smallsword. Considerably lighter than the blunted weapons we use on stage, and so sharp—he warned me—that if I rested the point on my shoe, gravity would be enough to run it through my foot. It felt dangerous, radiating menace like a loaded handgun.
The smallsword is the pinnacle of evolution in the western sword tradition. Imagine a ribbon of steel sharpened to a point as fine as a hypodermic needle on the business end but flaring out to a stiff, triangular base to maintain its rigidity. The smallsword has no cutting edge. The focus is on the deadly tip, designed to puncture your opponent’s blood bags—the heart, the lungs, the kidneys, the liver, the spleen—and spill the contents. But on stage, it doesn’t look much different from a fencing foil or epee. Having handled a real smallsword, I have a healthy appreciation for the danger they represent. And now you do too. But your audience doesn’t. You have to create that for them.
On stage we manage this by having the actors “sell the danger”—handling and reacting to their prop swords as if they’re as deadly as the real thing. If at all possible, I’ll introduce a wound early in the fight choreography to demonstrate the threat they represent. One of my favorite film fights is the opening scene from The Duellists, which I sometimes show to my stage combat classes. The victim’s panic sells the threat of the weapon (and allows Harvey Keitel to seem cool by comparison). 
Swords aren’t the only unfamiliar weapons. Terrorists threaten San Francisco with VX nerve gas in The Rock. There’s some exposition about how deadly it is, how many people will die depending on which way the wind blows, but even better, they show us, demonstrating the threat up front. there’s also a great example of demonstrating the threat up front. In an early scene, the renegade military men are “liberating” VX gas from a weapons depot and one of them, lagging behind, accidentally triggers the gas. The others, for their own protection, seal him behind an airtight door and watch him die an agonizing, eye-bulging, skin-blistering death. That’s what’s in store for San Francisco, including the hero’s pregnant girlfriend, if the terrorists win. 
Think you don’t have to worry about selling the danger because you’re writing about more realistic contemporary weapons? Guess again. Audiences have become inured to guns as well. I remember watching Christopher McQuarrie’s directorial debut, The Way of the Gun, in the theater when it first came out. He’s the guy that wrote The Usual Suspects, which, if you remember, was light on gunplay. For his followup, he wanted to create a commentary on Hollywood’s obsession with violence, a film so over the top it might possibly be the last such film ever made. (Spoiler: it wasn’t.)
At the climax, McQuarrie staged a massive gun battle between a dozen men armed with revolvers, shotguns and automatics. It’s slam-bang action. People are getting wounded, they’re getting killed. The film is a hard R, so it’s not exactly shying away from the graphic consequences.
In the midst of this, one of the protagonists spots a dry fountain in the middle of the courtyard. He makes a run for it and dives in.
Only to discover that the bottom is littered with broken bottles. 
The audience gasped.
It’s crazy—people have been getting shot left and right, and no one flinched, but then Ryan Philippe gets stuck with sharp glass, and suddenly everyone’s empathetic. Because we can relate. Most of us have never been shot, but we’ve cut ourselves on broken glass. We know that pain. This one scene does more to make McQuarrie’s point—that violence has been reduced to entertainment, devoid of empathy—better than anything else in the movie.
This is the bookend to the first article in this series where I argued that fights should be violent in the sense that terrible things happen to people we care about. Here I’m saying that your audience has to understand those terrible things at a visceral level. They have to be able to feel for your characters and understand their pain, lest you’re reduce to writing “action.”
1. The opening fight from The Duellists.
2. The raid scene from The Rock.
The actual symptoms of VX gas exposure, according to Wikipedia, are localized sweating, twitching and nausea, followed by asphyxiation due to paralyzation of the diaphragm. Not quite as dramatic (but still deadly).
3. The fountain scene from The Way of the Gun (fountain dive at 2:15 if you want to skip the gun play).