There are plenty of ways a horror movie can scare you.

A monster can jump out of a closet.

A killer can burst through a door.

A demon can crawl across the ceiling.

Or—and this is considerably worse—the person sitting next to you might not actually be human.

That last one is where Screamers and The Thing become particularly terrifying.

At first glance, the two movies seem very different.

The Thing is a 1982 Antarctic horror classic featuring a shapeshifting extraterrestrial organism.

Screamers is a 1995 science-fiction horror film involving self-replicating killing machines in a war-torn future.

One has Kurt Russell.

The other has Peter Weller.

One has a creature that can imitate humans.

The other has machines that can imitate humans.

And both have essentially the same nightmare:

You don't know who you can trust.

That's where the real horror begins.

The Monster Isn't the Scariest Part

The easiest way to describe The Thing is "Antarctic alien monster movie."

The easiest way to describe Screamers is "killer robot science-fiction movie."

Both descriptions are accurate.

Both descriptions also miss the point.

The monsters in these movies are terrifying because they destroy something humans desperately need in order to survive:

trust.

When you're trapped in an isolated environment, you need other people.

You need someone to watch your back.

Someone to help repair the equipment.

Someone to find food.

Someone to keep the generator running.

Someone to help you fight the monster.

But what happens when you can't determine whether that person is actually human?

Suddenly every potential ally becomes a potential enemy.

And that's when paranoia takes over.