Obviously The Role

"Don't judge a book by its cover" can suck it.

When you can tell what role someone might be by simply looking at them. No dialogue or backstories are needed, just knowing what their physical appearance is like is all it takes. This is partially due to how a character can have a cliched look to them, like how a man wearing a trenchcoat, sunglasses, and a shady fedora would easily be the Dealer and nothing else (unless our expectations were gotten). It's not just the appearance of the character that makes it easy to tell who they are — it's also their behavior, voice (if they speak), and the way they make other characters feel.