What shall we talk about today, kids? What kind of trope-itude shall I subject your tiny, yet awesome, minds to?
Ooooooooh. I know.
Let's take a field trip to the Uncanny Valley!
For those of you who didn't know that there was actually a NAME for this (and since you're on the internet, it shocks me that you DIDN'T), the "Uncanny Valley" is a robotics hypothesis that basically posits that a human replica that looks and acts almost, but not perfectly, like a real human being makes us want to drive away very fast, clutching at the dashboard and shrieking like a cheerleader.
The phrase itself was coined in 1970 by Masahiro Mori and is backed up, somewhat, by the work of Ernst Jentsch and Sigmund Freud. Freud's essay was called "Das Unheimliche". I mention it because it's funny to say. No, seriously. Say it in your head. Sounds awesome, right?
So, yeah. As an artificial being approaches human appearance, it seems to do OK until about the point when we have robots with faces that are supposed to be human. Then our love of our potential robot overlords goes completely to shit.
There are a bunch of theories that have been put forth to try and explain the phenomena. Scientists have posited that it could be anything from "Mortality Salience" (a fear of death, mutilation, loss of bodily control, or replacement) to religious concerns to the same violation of human norms that makes us fear clowns.
Now, for all intents and purposes, this is an entirely visual (and subjective) trope. It can be mentioned in literature or otherwise written about (for example, the Galateids in the New World Of Darkness game Prometheus: The Created are described as evoking this trope which is the primary source of the "disquiet" they exude.) but to get the full scope of this horror, you need to see it and you need something to compare it to.
There's this trend, lately, of grown women with entirely too much time on their hands and an inability to let go of a deceased child collecting what are called "Reborn" dolls. They look like this:
KILL IT WITH FIRE!!!
These lumps of cloth, plastic, paint and hair presumably let these woman cope with loss and, if it works, more power to them but DAMN they're creepy.
And this adorable real child agrees. He gets a cookie.
On the other hand, NOW, there are women who capitalize on the creepy and make dolls like this:
AWW!!! Look at her little baby fangs! ADORABLE!!
See what I mean? They've added a layer of utterly NON-human and it climbs back out of the valley. Only, hopefully, not really because we don't need some Twilight mom going all Team Infant on it.
Fuckin' creepy-ass Twilight moms. You know those are the same women that bitched about men being happy when the Olsen twins made it to legal age, right? Hypocritical bitches.
But, I digress.
In film, this usually haunts animation and it can be traced back to the rotoscoping used on Snow White and the Seven Dwarves. Trying to animate a real face over the real actions of a real person leads to some freaky looking movements. Just watch Ralph Bakshi's Lord of the Rings. Every one of those characters was cringe-worthy, regardless of how good the movie actually was. Nowadays, we have CGI and motion capture to contend with and it's almost always very obvious. This trope is the reason a LOT of people hated Final Fantasy: Spirits Within, Beowulf and Polar Express.
Sorry, sweet and avuncular Tom Hanks, but no. Just no.
In horror movies, we mostly see this in facial prosthetics (like the Wayans brothers' makeup in White Chicks) or in anything constructed to look human (which is usually stabbed or blown up half a second later). Chuckie in the original Child's Play evoked it because he was literally a doll. It wasn't until the later films, when he started to develop more human features like a receding hairline, that we weren't as squicked out by looking at him. In Evil Dead 2, we have Henrietta and the dancing skeleton of Ash's dead girlfriend. The nurses in Silent Hill were made to deliberately invoke the Uncanny Valley with their movements and human beings shot in stop-motion, such as in the remake of House on Haunted Hill, with their twitchiness were, again, deliberately shot that way to make people shit themselves.
But, there's a lot you can do to a normal human being to land them in the Valley, too.
Number one way to do it? Shave your eyebrows.
Eyebrows help us read facial language and when you remove them, we lose a good portion of our emotional connection with a person. Grima Wormtongue gave a whole lot of folks the wiggins in Jackson's Lord of the Rings.
Or you could just be a fantastic actor. Elsa Lanchester's utterly blank face, unblinking anime eyes and birdlike reactions make her DELIGHTFULLY creepy as the Bride of Frankenstein.
All of this being said, the Uncanny Valley is not a place I like to visit. Not one bit. I'd rather deal with unsettling towns with ancient, tentacle-filled secrets than try and interact normally with things I know aren't normal.
Not that I'm "normal" but you know what I mean.