Lesson learned from: Fright Night (1985), Shaun of the Dead (2004), This Is the End (2013)
What makes a horror movie? Its ability to scare the viewer, for one—that at times inexplicable way a film creeps under the skin, festers in a person's memory, and makes healthy sleeping habits hard to maintain. What's the polar opposite of "scary," then? The Return of the Living Dead Part II (1988), a ridiculous zombie misfire with several infuriating blunders. When a rotting corpse's severed, still functioning cranium gets a screwdriver driven into it, it says, with a hokey southern accent, "Get that damn screwdriver out of my head!" Later, a ghoul dressed like Michael Jackson for no good reason strikes a Wacko Jacko dance poses while being electrocuted:
See, that's not funny.
In addition being lazy, these gags are also instant credibility breakers. Even when a horror movie's trying to make you laugh as much as you cringe in fear, the horrific threats still need to be taken seriously. A great example of this is Shaun of the Dead—the jokes are all sharp and the characters are genuinely amusing, but whenever the zombies attack and/or connect en masse, it's convincingly imposing. The undead never smirk, nor do they act out of I-want-to-eat-your-flesh character. And, because of that, Shaun of the Dead has more than a few scenes that stand up against the best George Romero Dead film moments.