What makes a horror film scary?

Since it's the Halloween week, I was thinking about what makes people get scared. I am terrified of the sight of blood (the actual one, not in some film). So, I asked why do I fear cutting my finger by a knife? That question made me think more broadly on the duration of fear. 

Fear lasts as long as the action has not taken place. 
The uncertainty makes an experience scary.

​Fear lasts as long as the action has not taken place. The uncertainty makes an experience scary. 
Let's think about it. In a horror film, the director gradually builds up the story and as viewers, we are all waiting for something to happen. As we are watching the film, we start collecting these prompts of uncertainty (unconsciously). The fear starts rising as the moment of something happening gets closer. Scariness is at its peak right before the moment happens - a scary human comes and touches a person, a loud sound breaks the silence, among others.
For a film to be scary, there has to be someone who is getting scared. The two part approach of someone who is causing the fear (the ghost, say), and the person getting scared is what allows the user to connect the scared person to themselves and get scared. 

Back to why I fear the sight of blood. My uncertainty is that my finger will get cut and blood will ooze out. The fear lasts until that actually happens. I keep thinking of the knife getting close to my fingers, which makes the experience more fearful. That's because when the experience actually happens, it's done. There's nothing more to expect. The same holds true for fear of other things - travel accidents, job loss, relationship breakups, etc. There are several paranormal horror movies which highlight that.

I would end by saying that there's more to a horror film than the build-up. Sometimes, the director also adds moments which you didn't expect would happen. ​