Horror is about fear, and fear is about waiting. Once something bad happens, you’re not afraid anymore; you’re too busy being eaten. The story’s over!
So a key technique in horror is introducing something creepy into an otherwise ordinary scene, and then not rushing to show it fully, but holding back and drawing out the reveal.
Here’s Neil Gaiman doing this in Coraline. Notice how understated the writing is. Small actions. Simple words.
Also notice how he uses short, one-sentence paragraphs to draw out the sense of mystery.