I knew the apes in Dawn of the planet of the Apes are entirely made with CGI using the actors' facial expressions and body motion. Still, watching this side-by-side comparison between the motion capture work and the final VFX is still pretty amazing.
The process was invented and perfected by Weta Workshop—Peter Jackson's special effects company—for Gollum in The Lord of the Rings. Coincidentally, that's Andy Serkis on the top image, the guy who played the tormented Tolkien character.
On the other hand actors had to train really hard to imitate the movements of real apes.