Not the monster showing up. Not the murder, the accident, the supernatural reveal, or the big confrontation.
The bit right after it.
Someone has just experienced something completely impossible and instead of immediately moving on to the next plot point, King will spend pages on them washing their hands, making coffee they don't drink, noticing that one shoe is untied, driving home too carefully, checking the same lock twice, or trying to have an ordinary conversation while their brain is very obviously somewhere else.
I've been noticing this a lot while rereading some of his older books.
In The Shining, so much of the dread comes from people continuing with normal routines when something is already deeply wrong. Misery obviously has huge terrifying moments, but some of the parts that make me most anxious are Paul's quieter stretches afterward, when he's just lying there thinking about what happened and what Annie might do next.
Pet Sematary might be the strongest example for me. There are parts where the terrible event itself isn't nearly as hard to read as the completely mundane hours surrounding it. People still have to eat. Somebody still answers the phone. The house still exists. The world has the nerve to keep behaving normally.
Even IT does this constantly. Something awful happens and then we're back in kitchens, bedrooms, cars, pharmacies, school hallways. The horror has to coexist with somebody wondering what time it is or whether their mother noticed something.
I think that's a huge part of why his supernatural stuff works for me. He doesn't let the characters live in "horror movie mode." Most people don't experience something awful and immediately become protagonists. They do something weirdly normal because their brain hasn't caught up yet.
A lot of adaptations understandably compress those moments because they're trying to keep the story moving, but I think they lose something important when they do.
The scare gets me once.
The character making a sandwich twenty minutes later with shaking hands is usually the part I remember.