How do you make a story as scary as possible? The two things that I've been told is to make the story suspenseful by first making sure your readers know something terrible is going to happen, but not know what exactly is going to happen and when it's going to happen. The second thing is by making sure that the threat or monster has some kind of human feature and looks like a human but doesn't look completely human. I feel that comibint the two is not enough to write the scariest story possible. There's something like a secret sauce that you need to add to make it really scary, but I don't know what it might be.
Might be counter intuitive, but make it hopeful. Maybe I am a bit of a sadist, but when I watch a movie where I know the good guys will get slaughtered like lambs it desensitizes me and makes me laugh/bet on how long they will survive because the outcome is inevitable... and why fear the inevitable?
I'm currently playing a story driven game, I have been postponing the ending for two weeks now simply because I'm afraid the main character is going to die. If I knew her death was a sure thing I would have finished the game by now.
Additional make the viewer/reader care about the people who are in the danger zone. If the character is unlikeable people won't fear his death or pain but hope for it. If the character is likeable and innocent people would hope for a happy ending and fear the opposite.
And lastly make them relatable, a god like being in a (for him) dangerous scene doesn't really instill fear... a normal person just like the reader/viewer in a scenario where he/she would be in the same danger gives a better sense of dread.
One thing I've noted from reading horror (e.g. Stephen King) - the evil human characters are far scarier than any of the supernatural monsters presented. You fear for Rose Madder's life not because she's being chased by a werewolf, but a violent but well-connected husband.
Convince the reader that perhaps your story could happen in reality.
Not necessarily spiders; also snakes, skeletons, etc. If you want to make a story frightening, then evolved human instincts, as well as conditioned cultural fears, are your friends. What promises the most horrifying fate? A high Geiger counter reading, a ticking clock attached to something that might be a bomb, or a staring human corpse with flies buzzing around it?
Radiation poisoning is not a nice way to go, and finding out you're in the middle of serious radiation is probably what ought to frighten you most, but the corpse is likely the most viscerally disturbing thing, followed by the culturally-conditioned fear of what might be a bomb.
Lovecraftian horror, some kinds of ghost stories, certain philosophical propositions (Roko's Basilisk for some people, the Truman Show for others)...
Very peculiar things can be frightening to small children, who are not at all sure what is possible and what isn't. When I was very young, I was shown a cartoon of a "sewer monster" emerging from a toilet. I'm sure to an adult it was actually a silly picture. After being shown that picture, I got up in the middle of the night and put the heaviest thing I could on top of the toilet lid to hold it closed. (An empty ice cream bucket, used to hold bath toys, which I had filled with water. Yes, this caused problems for someone else.) The idea of something coming up from the toilet is terrifying - if you believe it can happen. Small children don't always know what is possible and what isn't, and that can sometimes be very frightening indeed.
If you can make adults unsure about what can happen, they might be frightened, too. What if the universe is full of awful, uncaring things which might torment you for eternity? What if there's something in the dark, waiting? This of course also plugs back into the "spiders" idea. Or rather, tigers. What if some hungry beast is lurking in the dark? Our instincts definitely err on the side of fearing that there might be tigers, or poisonous spiders, or whatever - because being too careful generally hurts you less than not being careful enough.
Body Horror and the Uncanny Valley
Some human reactions can blend into each other. If you can engage a person's imagination in grossing them out (what if your arm was cut off and a giant squirming maggot was sewn on in its place!), the sheer revulsion might be indistinguishable from fear.
On the other hand, that dissonance from things which are just slightly off can make people uneasy. Again, this can blend into fear. The photo-realistic face which doesn't move correctly. Hands drawn convincingly, but with the proportions slightly off. A body moving like a puppet, and not fluidly like a person. Your mileage may vary, but small inconsistencies which tell your audience something is wrong, which they may not even be able to pin down, can add up to an uneasiness which contributes to a fearful uncertainty.
The difference between an adventure story and a horror story is helplessness. The moment your characters have a fighting chance to actually overcome the monsters, you are telling an adventure story. An adventure might be scary in places, but the emphasis has changed. "We can do this!" naturally contrasts with a sense of dread. What makes a nightmare a nightmare (well, for my nightmares, anyway) is the certainty that if the thing catches up, it's all over. At the end of Invasion of the Body Snatchers (the black and white one), it stops being a horror story in the last few minutes, after the accident with the truck which spilled a bunch of pods. Suddenly, it was the military mobilizing to fight an invasion - not the growing dread that falling asleep might mean the end of you, and your replacement with a copy just like you but working against everything you care about.
Alien also experienced a turn at the end (after including "Spiders", Uncertainty, Body Horror, and Helplessness), where the final crewmember successfully fought back to overcome the alien and kill it.
The Fine Line
When I was talking about uncertainty, I mentioned my childhood fear of the toilet monster. But it's comical now that I have more life perspective. If you attempt to inspire fear through uncertainty, sometimes you just get amusement. If you attempt to inspire fear through revulsion, sometimes you just get revulsion. And some people aren't afraid of spiders or snakes.
Trying to take something scary and make it more frightening may tip you into campy silliness, dull intellectual abstractions, or stomach-churning (but not frightening) gore... Scary stories cannot always be made more scary by doing more scary things. If your reader loses engagement with the story, because they don't care about the characters, or your list of awful things is tediously long or absurdly implausible... Then you've gone past the mark.
There is a fine line between frightening and merely repellent (or uninteresting, or silly). And that line will be different for different people.
It's all in your mind.
What did I watch recently? It was a Scandinavian thriller, I think. Anyway, instead of showing us the slashed-up bodies, the film showed the detectives who were looking at the pictures. They were fine actors, and it really got the message across without the sadism-porn. At the same time you made your own pictures in your head and this was actually worse.
Ditto a book I read once. "Flight of the storks" by Grange. The hero finds the dead body of his mentor in a stork's nest. Instead of a description, he just says; "well, as you know, storks are carnivores." That's just - horrifying.