Movie Rule 9,145: All primates walk on their hind legs, show big, toothy smiles frequently, and give the thumbs-up sign.
Anyone who knows anything about primates knows that all three of those behaviors are completely unnatural to primates. No primate, including the chimpanzee, walks on its hind legs in the wild. Primates prefer to walk on their hind legs with support from the knuckles of their hands. This was particularly obvious in Every Which Way But Loose, where they had an orangutan with incredibly long arms that had to walk with its arms straight up in the air or hold onto a human to maintain its balance. As far as their smiles, showing teeth is a sign of aggression among primates, so they would never smile on their own. And if I have to explain why the thumbs-up signal isn't natural to the primate, then you need to get out more. But unless the movie is a serious portrayal of primates like Gorillas in the Mist, you'll see primates running around on their hind legs, smiling, and giving the thumbs-up, even when they're fresh out of the jungle.
Examples: Every Which Way But Loose, Planet of the Apes, Monkey Trouble