What is the secret formula for scaring someone [No cheap scares!]

[Kreekakon]

Ok, then here's what I think. I'll use SCP: Containment Breach as an example. It is one of the scariest games I've ever played, and here's why I think so:

1. The game is randomized everytime you play it. Not only does this increase re-play value, but it also makes the player not to know what to expect, keeping them on their toes. Regardless of whether your game is randomized everytime, this is important: Don't be predictable.
2. The art of anticipation, as stated before. Knowing SCP 173 is out there for you, but not knowing when he will strike makes the experience ultimately scarier.
3. SCP 173's "calmly" dangerous nature. The appearance of a seemingly calm thing makes it all scarier when the player still realizes that it's very dangerous. This aspect shares some elements with "fear of the unknown" in my opinion.
4. Ambient noises, and music can further intensify the player's fear coming from the anticipation.