Like the Rule of Cool and the Rule of Funny, the Rule of Scary says that if it's creepy enough, it doesn't matter how illogical it is. All that matters is that it creeps you out.
Most effective formulas for scary:
Mixing the paranormal or abnormal with the mundane and familiar. Even if the Willing Suspension of Disbelief remains intact, blatantly scary things are easy to shrug off when they are happening in a completely unrelatable situation. But when the same elements are placed side-by-side with familiar settings and elements, suddenly The Fourth Wall Will Not Protect You. Even with flawless special effects a Starfish Alien in an Eldritch Location can be mere Narm, but invoking the Uncanny Valley with a Humanoid Abomination in an Abandoned Hospital can be downright terrifying.
Ending on a note or element no sane person would. You'll find humans are psychopathic. What is funny, notched up, is scary (and still kinda funny). Exempli gratia: "How about a magic trick? I'm gonna make this pencil disappear..."
Vividly showing (or describing) something scary which directly affects the human body. Body Horror is a part of this, but this is not always Body Horror. eg, The Matrix using humans as "living batteries", or some portrayals of viruses. Artistic License – Biology is a major factor in this.
Not showing anything, and leaving some things unexplained. This works on the (fairly reliable) premise that no expensive special effect or clever writing can ever top what the viewer imagines to fill in the blanks. Used well, this method creates suspense, enhances the ultimate reveal, and can magnify a simple Jump Scare into a Brown Pants Scenario. Often combined with Hell Is That Noise and Obscured Special Effects. Offscreen Teleportation, Unreveal Angles, and implausibly agile monsters using Air-Vent Passageways are all classic methods of keeping the scary unseen as long as possible. The plot aspect often involves His Name Is..., Conveniently Interrupted Documents, Apocalyptic Logs, and/or Plot-Based Photograph Obfuscation.
Just making something mindblowingly scary and showing it in all its well-lit horrific glory — the hardest to pull off, as it risks highlighting even minor Special Effects Failures unless done superbly, and often has to overcome the lack of menacing atmosphere. On the rare occasions it is done well, however, it can be absolute Nightmare Fuel, all the more jarring because both viewers and characters tend to associate well lit areas with safety — which is not always the case. Numerous particularly well-done instances of this form can be seen in The Walking Dead, with walkers as likely to attack in open fields in broad daylight as in dark basements, making for prime Paranoia Fuel.
If you cannot tell from the listings below, this is overwhelmingly a Film trope. Whether this is because producers don't care about silly things like logic, or they just don't have enough time to explain every little idiosyncrasy, is a debate that will rage on for as long as the medium exists.
See also Hand Wave, Nightmare Fuel.
Compare Fridge Logic. Offscreen Teleportation is a Sub-Trope of this, as is Body Horror.
Contrast Narm, Nightmare Retardant, Rule of Cute, which negate a work's potential for fear. Also contrast Realism-Induced Horror, which seeks to instill fear in audiences through plausibility.
