There are many things that can scare people in scary movies such as:
What we can see but the cast cannot - Some of the scariest footage in scary movies can be when we see the psycho, monster, killer... etc creeping up on thier victim and there is nothing as a viewer you can do about it.
Ideas To Achieve This - Zoom in on the victims face and have the scary character some distance in the background out of focus and have them slowly come forward and perhaps come more into focus as they approach. Have the scary character crawl across the ceiling right above the victim, this would be effective if you used a low camera angle then the viewers would be able to see the scary character and the victim would not. Another technique you could use would be to film as if you where looking through the scary characters eyes, this is effective as the audience knows the scary character is creeping up on the victim but there is nothing they can do.
Disgusting can be scary - Some of our worst nightmares can involve mutilation and while in some movies it seems a bit random it is never the less scary to watch. Limb loss, impalement, dismemberment etc are all scary and freaky to watch. It is important in a movie not to do this to much otherwise it gets to disgusting to watch.
Ideas to Achieve This – Have the character open a door to somebody standing there with a severed body part. The person the character is talking to or is standing next has been murdered (decapitated) and they do not realise.
What we can’t see but can hear – With the good sound systems in most cinemas the use of sound is usually extremely effective. The eerie music that builds tension is extremely effective especially when nothing overly scary is happening. By adding scary music to an innocent scene it can completely transform the meaning of the scene. Also sound effects of what is happening off screen can also be very effective, for example when a character is being brutally murdered it is sometimes very effective to just hear it as apposed to seeing it as it is still just as brutally but keeps the film from being to gross
Ideas to Achieve This – Have the scary character grab someone and you see the action of killing (for example an axe being swung down at somebody) and just have the noise of it. This would mean the viewers would still know exactly what is going on but doesn’t have to view the gore and blood. Also another effective technique is to have a victim hiding in a closet or behind a door and just to hear the scary characters footsteps coming towards the victim, not only is this scary it also builds extreme tension.