This clip from 30 Rock is great. It simply describes the phenomenon of the Uncanny Valley. This is why going for realism in CG is very difficult. The closer you are to real, the more you plummet in to the valley of repulsion. Even if you nail something as looking 100% perfect human reality. There is a lack of emotion and a connection to the real that the effect lacks, and thus we feel as there’s something not there. Even in the best moments of photo real CG like in Beowulf or Final Fantasy, it was still uncomfortable and unnerving.