The Uncanny Valley is part of a thesis by Doctor Masahiro Mori which “represents the point at which a person observing the creature or object in question sees something that is nearly human, but just enough off-kilter to seem eerie or disquieting.”
The article is targetted at SF and Fantasy writers but I couldn’t help being struck by how much the same ideas relate to puppetry. The article suggests that movement is more important than simple appearance in affecting our comfort level when encountering something non-human.