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

[Damascus]

I think to make something truly scary you need to perfect the art of anticipation. Create an atmosphere that makes the player afraid to peek around the corner. A sound coming through the walls that make the player fear what might be on the other side. A sudden change in the music that makes the player wonder what's next.

Then, a good escalation on that is making the player feel helpless. Trap the player in a well-lit room as a grunt comes after him, or force the player to wander in a patrolled area to find an item.

There's plenty of things to experiment with, but I find anticipation and helplessness to be the two most critical things in making a good scare.