Usually Judge a Book By Its Cover

Mainly used for the fact that appearances have to make sense sometimes. It is important to not judge by appearances but even then there are some times where the good guy actually needs to look like a good guy, and the bad guy needs to look like a bad guy.

I'm just saying would you ever guess some hideous and grotesque behemoth is the hero and some beautiful and kind spoken princess is the villain? Honestly, you should usually judge a book by its cover.