But we're not here to talk about normal people, are we?
You didn't think this was gonna be all schmaltzy, did you?
In horror films, I certainly hope that it does not surprise you to know that people are fucking insane. We're not talking "take a pill and call me in the morning" insane. We're talking "bleached bones arranged in a fancy-schmancy tableau to simulate entertaining dinner conversations".
Welcome home, Daddy!
Yeah, folks in horror fiction tend not to want to let go. And, yes, I said "fiction" because this trope goes back to AT LEAST 1609 with The Revengers Tragedy where a character has been carrying around his dead wife's skull for 9 years and having delightful chats with her. In fact, I just lied. This trope is older than dirt. In The Epic of Gilgamesh, Gilgamesh won't allow his friend Enkidu to be buried for a week, treating him as if he were still alive. This changes when he sees a maggot fall off of Enkidu's nose, thankfully.
The deal is that these folks are so far around the bend that they usually don't even realize that their loved one is dead and, in fact, have developed their own twisted little reality in which that person is not deceased. They've just lost a little weight, that's all. The smell? What smell? I don't smell anything do you? Doesn't Mother look lovely in her Sunday hat? SHE'S NOT DEAD, STOP SAYING THAT!! You're just jealous! That's what you are. Mother says you're jealous and you're trying to ruin me with your vicious gossip! You'll have to be... dealt with...
You're ugly and your mother dresses you funny.
This is different from just talking to a dead person and not expecting an answer. For this trope to exist, one of the participants has to be a corpse which has not been interred and the other has to be completely fucking binkers and legitimately believe that the corpse is still living and breathing. This person will, for all intents and purposes, appear to be completely normal to those who are not "in the know" about their private arrangement. Usually, we'll end up watching them grab a sharp implement when the clueless pretty girl stumbles on the corpse by accident.
The most obvious example of this trope is, of course, Psycho, but it shows up in some unexpected places.
Technically, this appeared in Snow White, where the prince KNEW she was dead and fell in love with her anyway and just HAD to plant a sloppy one on the pretty dead girl. Also, technically, this appeared in Harry Potter. One of the Deathly Hallows was a stone that allowed a person to speak with the dead and madness was often a result of ownership since the holder would become convinced that the deceased were still alive. Ms. Spink and Ms. Forcible do this with their dogs in Coraline when they die (and then dress them up in little angel outfits because wacky is as wacky does).
In terms of just movies, though, we're exposed to this a lot. Not quite "always humorous" levels but enough to where we know what's going to happen . The AMAZINGLY gorgeous film Perfume has Grenouille becoming obsessed with Plum Girl's scent so when he accidentally kills her he hides her away and constantly smells her until her scent is gone. May has our heroine making a Frankenstein-esque doll out of those parts of people she finds to be perfect. Terry Gilliam's controversial Tideland gave us a little girl who taxidermies her drug addict dad so she won't be alone. (Fucking Jodelle Ferland... STOP BEING CREEPY! You'll just end up marrying Marilyn Manson and that won't be gross at ALL or anything!) And, for the fans, Texas Chainsaw Massacre 2 has the whole family treating Grandpa as if he's ready to carve the Thanksgiving turkey. (Edit: I have been reminded that Grandpa is not actually dead but he may as well be. I swear if that motherfucker tried to slip me a twenty the way my great-grampa did I'd scream like a little girl and run... then run back and snag the cash and say "Thank you, Grampa"... and then run away again, howling into the night.)
Here's your heroin, Daddy! Now go get dressed for dinner.
Do I really have to tell you why this scares people?
Fine. But don't say I never did anything for you.
This bugs people because we don't like to deal with corpses and, frankly, this can actually happen. It is nowhere NEAR outside of the realm of possibility that a person can develop an obsession over a loved one they are not prepared to lose and completely ignore the reality of a rotting human body to satisfy their emotional needs.
There are numerous examples of people keeping dead husbands, wives and boy/girlfriends around for a little post-life nookie (eew) and there was one highly publicized case of a family keeping the mummified corpse of a stillborn baby in the house (but that was more of an heirloom sort of thing and while they playfully included it in household activities they didn't actually believe it was still alive). Even that asshole Rick Santorum took a stillborn home to introduce to his kids.
On the other hand, the definition of "loved one" tends to get tossed around willy-nilly among the insane so a lot of times the crazy person is also responsible for the corpse's current state of being. Serial killers Dennis Nilson, Ed Gein and Jeffrey Dahmer all decided that they didn't want to be lonely no more and kept a few dates around. Dahmer, of course, had a different definition for "dinner and a movie".
Hang on a second... what? Oh. OK. Coming, Mother!