Val Lewton invented the jump scare in 1942, but that's not what he called it. He called it the Bus, and this scene from Cat People is why he called it that.
Let's analyze the scene.
It begins in ordinary circumstances. The character is doing something she does every day, and in a place where she would consider herself safe. The audience sees the threat, but she does not. She becomes isolated. She becomes aware of the threat, though she isn't certain where exactly it is. At the point where everyone - the character and the audience - expects the worst, salvation appears. The character survives the scene, though we are made very aware it could have gone differently.
This differs from the modern jump scare in several ways.
First, the modern jump scare depends heavily on atmospheric circumstances, specifically music. We know when those strings thrum that something awful is coming. There are strings here too, but not that blaring unsubtle BZAAA. If anything, the music is much louder before she goes into the park and dies to nothing once she enters it. Instead we hear her footsteps echo in the dark - and possibly the echo of someone else's footsteps. No music at all.
The hiss of the doors opening so closely mimics the sound we expect to hear - the big cat's scream - that for a split second we're not sure whether we heard the bus, or the cat. Whereas in a modern scare there's never any attempt to confuse; it's always BZAAA.
However we see a great deal, and that's another difference. Often in a modern jump scare we are allowed to see very little. It's a dark corridor, a dark spaceship, a dark house, with shadows blocking everything. Or the camera is fixed and unmoving. Or the camera is tight in on the protagonist, allowing us to see very little other than the protagonist. Yet in Cat People we see everything, often from the protagonist's viewpoint, and specifically we see her dart from pool of light to pool of light. Yes, there is darkness between the pools but it does not obscure the entire scene. If anything it tantalizes by allowing the audience to see just enough and no more.
Cinematographer Nicholas Musuraca earned every penny of his paycheck. It probably helped that he had complete control over his environment - the whole thing's shot on a studio lot. They built that park from scratch. Mind you, they built it so much like Central Park I doubt anyone familiar with it in 1942 could have told the difference.
Finally, the greatest difference is that in a jump scare we often discover that the threat is fake. In this, we know the threat is real. We know, and the protagonist knows, that there really was something out there. We just didn't see it catch her.
It begins quietly, spikes the tension, SPIKES the tension, SPIKES THE TENSION and then … leaves us unfulfilled but terrified. Because we know that although the threat didn't follow through this time, it was a real threat, and it is still out there, and it will try again.
This isn't the only Bus in Cat People, but Lewton knew that overuse of the technique would deaden its effect. You have to earn your Bus; you can't just leap on screen and hope for the best.
So how does a horror Keeper do this at the table?
To begin with, let's set the scene. Assume this is part of the Dracula Dossier. Assume it is taking place in Whitby. Assume it takes place at nightfall or early evening.
This shall be a transitional scene. The agent has completed a task and has to get from wherever the agent currently is to the meeting place or safehouse, and discuss the events of the day with the other agents.
The Dossier gives several Emotional Modulation moments. It's useful to have a few of these on standby for any game. You don't have to kill yourself planning these out; be brief, evocative, and don't nail down the details since those will change depending on circumstances beyond your control.
A sudden rainstorm breaks. You're drenched in seconds and huddle briefly under the patched canvas of a closed shopfront's awning, icy ropes of water slithering down your back and neck. There's a brief pause in the squall, and you think you might have time to get to where you need to go.
The agent either moves off or stays where they are.
You catch a glimpse in the reflection of a window [shop window, car, something else].
The player will probably ask, "glimpse of what?" Don't answer. Don't be drawn, don't add detail. The player will be adding all the detail you could ask for, so long as you let imagination run riot.
Spike the tension.
The agent probably moves off at this point, but might try something else.
Your footsteps on the wet pavement are the only thing you can hear. At this time of night there ought to be people about, surely? You haven't passed a single person in the last five minutes. The streetlights go on, one after the other. The one ahead of you has failed, as has the one ahead of that.
Tone of voice is important here. Be very calm and deliberate. Take your time. The more agitated the player becomes, the more tranquil you should strive to be.
SPIKE the tension.
There's definitely something behind you. It's out of sight but you can smell it, something rank and rotten clawing its way down your throat, gagging you. That, and the copper stink of fresh blood.
SPIKE THE TENSION.
A door opens just to your left, and a startled shopkeeper blurts an apology for almost barging into you. The streetlight next to you flickers on. She mutters something about the Council and starts her journey home.
[The agent may try looking for the source of that smell.] You finds a pool of fresh blood next to a shopfront, not diluted by rain so it must have been put there moments ago. By what, you can't be certain. [Tests, if carried out, show it to be animal blood.]
There's no indication how it got there. It's as if the creature vanished altogether, whatever it was.
So what happened exactly? The agent began in ordinary circumstances. Players are creatures of habit; if they get used to things happening in scenes, and they just had a scene - investigating Whitby Abbey, say - then they won't expect anything to happen 'out of scene' or in a transitional moment between scenes.
The setting was normal. Even if your players have never been to Whitby you can show them pictures - God knows there are plenty of them. They can see in their minds where they are. At the same time it is not precisely as they picture it. Where are the people? The place ought to be packed with tourists, residents - did they all vanish?
Again, tone of voice is important. You are striving through your performance to achieve the same effect that Lewton and Musuraca did with camera shots. There is no sudden jerk, no zipping about the place. The shot is calm, unnaturally calm. You can see everything - and imagine worse. It all looks fine, which is precisely why you're frightened.
Not answering questions is as important. Players are used to you answering questions. It gives them comfort, and the last thing you want is for them to have comfort. You want them not sure of their environment, their immediate surroundings, or the true nature of the threat. The only thing you want is for them to be absolutely certain that there is a threat.
The unnatural enters the scene. The agent could smell it, knew it was close, but could not see it. That's important. If they can see it they can attack it, try to kill it - overmaster the threat, in other words. An intangible threat is different. You can beat an enemy standing in front of you. You cannot hit an enemy that exists only in your mind.
Or, to put it in D&D context, if you give hit points to Cthulhu then the players know they can kill Cthulhu, if they try hard enough. So don't give Cthulhu hit points.
Then comes the release, and it's important that it be an ordinary release, bringing the mundane back into the scene with a crash. Once that shopkeeper opens that door, the unnatural takes flight.
Yet we know the threat is real, else why the pool of blood? It must have been close, and it must have been there only a moment ago, otherwise the rain would have washed it away.
No points were spent. No tests were made. The player can call for tests, of course, but that's a different thing. The Director didn't insist on it.
Now the agent's caught Lewton's Bus, let the agent ride it for a while - nerves jangling all the way.