Sometimes, you can tell from a character's first moments exactly who and what they are. A character appears on-screen and immediately kicks a dog, he's obviously the villain. Another appears and his first act is to help a kindly old lady across the street, he's a good guy.

But wait... that guy wasn't actually kicking the dog, he was stomping on a spider that was about to bite it! And that so-called good guy was only helping the little old lady so he could steal her purse once her guard was down!