The unknown. In many of the stories like 'The doorbell rang at 3 am. No one was there when I opened the door.', not knowing who or what caused the doorbell to ring is quite terrifying. It plays on our fear of the unseen and unexpected. There are often no explanations given, which leaves our minds to fill in the blanks with the most horrifying possibilities.
The isolation factor. When a character is alone in a situation, such as 'I was home alone when the lights started flickering. I knew I was not the only one in the house.', it heightens the fear because there is no immediate help available and the character is vulnerable to whatever threat is lurking.
The suddenness. For example, in 'I saw a shadow on the wall. It moved without a body.', the idea of a shadow moving on its own is very sudden and scary.
In the story about the teddy bear, the scariest part is the sudden transformation of a cute and innocent teddy bear into something with human eyes. It's a complete subversion of the normal, comforting nature of a teddy bear.
In some, it's the sudden appearance or disappearance. For example, 'She was walking in the empty hallway. A figure emerged from the wall and vanished.' The unexpected emergence and then quick vanishing is really scary.
The sudden appearance of the unknown is really scary. For example, when a figure just flashes by at the corner of your eye in those stories.
In the first story, the scariest part is when the boy saw the red eyes staring at him from the attic window. It was a sudden and menacing encounter. In the second story, the cold hand grabbing the girl's ankle was terrifying as it was unexpected and she was all alone. For the third story, the dark figure lunging at the family when they opened the hidden door was the scariest moment as it was a direct threat.
Well, the one where 'I woke up in a coffin. As I screamed, I realized I was buried six feet under.' is quite terrifying. Just the thought of being trapped alive underground is a nightmare.
Another really scary one could be 'My phone rang at 3 am. The caller ID said it was me.' It plays on the idea of the self being something other or unknown, and calls at that time are always spooky.
The power of their jaws. Crocodiles have incredibly strong jaws that can crush bones easily. In a horror story, when they clamp down on a victim, it's a terrifying sight. It's like the story of the tourist dragged into the swamp. Once caught, there's little chance of escape due to that crushing force.
The scariest part is often the unexpectedness. For example, a virus can spread through the network in a matter of minutes and paralyze the whole system before anyone realizes what's going on.
The scariest part is often the unknown effects on the brain. For example, when a patient experiences sudden and unexplained changes in cognitive function after a TMS session, like memory loss or confusion. It's terrifying not knowing if these effects will be permanent or reversible.
The unknown is often the scariest part. In Citadel horror stories, it's things like the unexplained noises. For example, a sudden creaking sound in an empty corridor that seems to have no source.