Don't get me wrong: I firmly thought the aliens from A Quiet Place were quite terrifying to look at. Lots of monsters are. Honestly if you can't just stare at these heinous beasts without trembling even a little bit, then you know they're doing the job right. They're scary. Bravo.
What's Scarier Is What They Might Do to You
Or better yet, what you've seen from what these monsters have done. This is where terror is born. It's the key seed of fear planted at the right moment to get you wondering just what is this horrible thing plaguing the town? A vampire? A leech? An alien from outer space? A feral candy bar?
The questions planted in your mind do all the work.
We've seen it all the time in classic works, actually. When you don't see what that monster is, you then wonder even more at just how hideous it might be. Remember the film Bird Box? Truly terrifying in a cerebral way, and it was quite fitting: because you don't actually see what these winged monsters looked like.
All you see is what they do to everyone. And that's horrific enough. Those beasts made you either go crazy or kill yourself.
Perhaps you see a hint of it, or a shadow. You play on the size or the obscure, and you get the mind curious, like the stereotypical character in a teen horror flick wanting to check the basement for some dumb reason. When your reader or viewer ends up asking lots and lots and lots of questions even before the big jump scare, guess what: you've done the dark deed well.
So Here's a Little Word of Advice: Don't Worry About What Your Villains Look Like
At least not right away. Let the actions of your villains speak for themselves. Let the dark deeds do their worst in the minds of your readers or viewers, and let that seed grow into the worst carnivorous plant in all of history. Then when your reader/viewer least expects it, unleash the horror! And reap the reward.
That's just one tip for you. Ready for more? Take this quiz, and let's see if you're ready!