Production
Uncanny Valley
The phenomenon where a computer-generated figure or humanoid robot bearing a near-identical resemblance to a human being arouses a sense of unease.
In Depth
As a character becomes more unrealistic, we find it cute (like Wall-E). As it becomes *almost* human but not quite perfect (dead eyes, stiff movement), our brain rejects it as a 'corpse' or 'sick', causing revulsion.
Example
"The animated characters in 'The Polar Express' are often cited as prime examples of falling into the Uncanny Valley."