Character and Narrative: Uncanny Valley

Uncanny Valley is a phenomenon that occurs when features look and move almost but not quite exactly natural which can cause the viewer discomfort. The concept was first identified by a robotics professor Masahiro Mori in 1970. He proposed that as the appearance of the robot becomes more human some observers' responses will become more positive, until it reaches a point (known as the valley) were the response will rapidly change and "become that of strong revulsion", and then back to a positive response once the robot becomes "less distinguishable from that of a being".

Example Graphs to Explain The Uncanny Valley

This concept can also be quite common in Computer-Generated Animation. A good example of where this has occurred is The Polar Express. The film was released in 2004 and received mixed reviews, with many stating it was visually stunning though the film dropped in ratings due to the character design. The characters felt "lifeless" and "dead behind the eyes" which made the film feel quite creepy/eerie.

The Polar Express - Movie Trailer

Some people believe that the Uncanny Valley simply represents the limit of our ability to create realism, so one way to avoid creating discomforting characters would to be to avoid using overly detailed motion captures or textures to unrealistic models, but instead using a motion that matches the realism for your "level" of animating. Keeping an eye on the eyes and facial expressions is also a good way to avoid plunging into the Uncanny Valley. Instead of striving for detail, make sure that the face expresses emotions so that it seems as though there is a conscious mind and life for the character, which is what went wrong for The Polar Express.
This life that you create also needs to be accompanied by the correct voice as characters can become uncanny if they have a voice that clashes too much. For instance, Cartoon characters tend to be quite unrealistic and exaggerated so they will be able to "get away with" an unrealistic voice to match, yet if a realistic character had a voice like this it wouldn't work in the same way. Instead it would remove the ability for you to relate and empathize to that character.