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

[Kman]

Yeah build up is really the way to go. The anticipation of an act tends to be much more scary then the act itself. Say right now some psychopath starts breaking down your door trying to kill you, you'd probably be shitting your pants in absolute fear. But you know what would be worse? Hearing your front door creak open, heavy boots going across the floor and then suddenly cutting out, especially if say you were home alone and no one was expected to be back any time soon. At this point you'd be absolutely terrified. You don't know who it is, for all you know it could actually be someone you know coming by the visit. But you can't tell for certain, and THAT'S what makes it so scary.

In my mind almost all fear stems from fear of the unknown (as was stated in the OP), you don't know if that noise was some maniac that's come to kill you or if it was some ordinary every day noise that you just can't think of at the time. Really that concept can be applied in some way or another in any place where you feel scared.

As for your question, tension is hard to balance, but the main key is you can't let the player feel stuck or familiar with a place. What I mean by this is the longer you spend finding out a puzzle in an area, the more time you spend back tracking, etc., the less scary or tense it will be. This is when the player will start to lose interest. This was actually one of the problems I had with the Jail in Amnesia, I spent so long trying to find my way around there that I stopped being scared and started feeling a tad bit annoyed. When you begin to feel aggravated you tend to lose immersion, which leads to you no longer being scared.

Really the way to avoid this is to make sure the player can't get stuck in a tense area, or at least make sure if you know think they might get make sure to have some areas unexplored, that way you never do know if there's some monster waiting behind it or not.

Hope this was helpful, and I swear to god if this turns into another stupid arguments I will slap a small child. srsly.