Michael Grimm, a favorite commentator of mine, mentioned that one of the key elements of horror I forgot to mention in my immersion thread was building tension. You know, he was absolutely correct. Immersion is one of the best ways to build up horror, making it so nothing snaps you out of your suspension of disbelief. However, arguably more important, is the ability to build up things to a crescendo.
Then take your audience well past the point they're ready for a climax and make it so much worse!
No, seriously, tension is the process of building suspense and getting the audience invested in waiting for something to happen. This could be our heroine heading downstairs into the basement we know a serial killer is waiting for or noticing the fact her child is possessed.
One of the reasons I really like the Marble Hornets series is that it's very good at building suspense. It's why the otherwise silly premise of looking for the appearance of the Slenderman in split second frames of the movies or for him to show up at the end is so terrifying. Nine tens out of ten, the Slenderman doesn't even do anything but we're invested in his appearance to the point we're on the edge our seats waiting for his arrival.
Which is, oddly, why I hate jump scares. Jump scares are those moments where the tension has built to a crescendo and then out of nowhere something happens. Either that or, worse, there's no tension and something happens! Aren't you scared? Well, honestly, no, no I'm not. In the first case, breaking the tension is a bad thing unless you really want to sell it. The second feels legitimately lazy. The "cat scare" is my most hated example of this, you know where the tension becomes really thick only for a noise and oh it's a cat (usually before the monster attacks).
I hate this. Don't ask me why.
One of the greatest of all sources of tension in movies was, of course, the build-up to the events of Psycho. Alfred Hitchcock, it's not a surprise to say, was a master of tension even when he wasn't doing horror. The heart of the movie is the audience building up sympathy for its main character, who is not Norman Bates, coming to like her and then becoming increasingly aware of how much danger she's in. When the movie killed their "lead" halfway through the film, the audience had no idea what was going to happen from that point on and everything continued like a roller-coaster despite its most likable character being dead.
Jaws and Alien are two "monster movies" which benefit also from the sparing use of their monster. They appear just often enough that their presence looms over everything. Too much of a monster and there's no real fear from it as you have the rest of the story to fill up with him so too much damage is impossible. Just enough, however, and they can lurk over everything. The terror is in the waiting for their arrival. Of course, you do need some appearances to remind audiences why they should be afraid. I've seen a few books who don't give enough of the monster to make the audience afraid for our heroes.
In general, tension is like a secret the audience has been let in on. They're aware of the danger and so is the storyteller but the protagonist is not. Even when the protagonist is made aware that Jason Voorhees or whoever is in the woods, they should never know where exactly. Sometimes, it's a good idea to have the audience have some breathing room. That way they can be scared all over again.
What is the perfect mixture of appearances versus not for creating tension? Difficult to say because I think everyone has a different threshold for it. For me, I like tension that almost never breaks. The build-up where the audience knows the villain, knows something is going to happen, and the movie strings it along even when it should be giving you time to breathe. For me, the resolution is almost an afterthought.
Just my .02.