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. Some roles go the opposite direction of Obviously The Role by using foolery that could work in-universe but doesn't lie to the audience at all. Two roles that this can apply to are the Mega-Cutie and Cuddly Killer. If someone is being oddly sweet and cute, the audience and Brains can see straight through their guise and sniff them out instantly. Everyone else shouts the Brains down and calls them out for being the Lunatic, but this doesn't stop them from keeping their eye on the Suspect in what's likely a Chicken Little plot.