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

[Adrianis]

A good story also has to be creepy like, you are doing something you dont want to. Kill a man to get past something...

Or you watch something without having the ability to stop it.

And always: Anything unknown.

There is something, or is there?
Is it following me? Did i see it right there?
Am i imagening it?
What was that?

Your last point, totally agree. Nothing you will see in game currently will amount to what fears you can conjure up with your mind, as long as you leave gaps to fill.

Case in point, when your going through the tunnels early on in Penumbra, when you first come across the dogs, when I got warned about how to deal with enemies, it was suggested that an enemy was around. With the utter darkness and the atmospheric music, when I heard the growling and didn't know what it was, I was terrified enough to fear-quit. Half a year later when my balls had dropped again, I went back and saw what it was, and promptly beat it to death with a hammer, and from then on could cope. Sadly it only took a few more hours for me to get stuck on a puzzle an quit again, because of which I still haven't gone back, so I never got another chance to be terrified by a new enemy.

The problem with having the player do something they don't want to, from a moral standpoint, such as kill an innocent human, is that you need to get the player emotionally involved with the world and the character, otherwise its not going to matter to them and it won't have the desired impact.