A classic example: in the original "Halloween", there was no motive for Michael Myers' actions, making it a lot more creepy, as you couldn't guess what was going to happen next, and it made him a lot more sick.
But they gave it an odd explanation in the sequel, which may have helped start a plot thread for the franchise to grow on, but reduced some of the horror.
"Night of the Living Dead" was a lot more messed up when you didn't know why the undead were leaving the graves. Then Romero attached some explanation about a comet. Luckily, no explanation was attached for "Dawn of the Dead"...
In the first "Jaws", there was no explanation for the shark's behavior. Then it became some odd "revenge" theme throughout the rest of the franchise, which made it silly...
People fear more that which they do not know and cannot explain. Leaving out some details also increases the probability that it could happen in real life...
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