I think it must be the whites of the eyes. On the left, a real chimpanzee. On the right, a fake ape, from Rise of the Planet of the Apes. The film is a slick, entertaining if rather obvious morality tale for our times – and left me thinking how they best conveyed the smart minds of the altered apes.
There was the advanced use of tools, the signing, and eventually even speech. But I think the most convincing moments were mostly achieved with the whites of the eyes.
(Image left: Common chimpanzee in the Leipzig Zoo, by Thomas Lersch)