People make assumptions.
Whether we want to or not, when we see a person, we make decisions about them immediately. These are based on our own experiences, and things that we hear and read. A complete stranger might remind us of somebody who plagued us in high school, and we react with visceral dislike. A different stranger might remind us of a beloved grandparent, and we receive them warmly.
Sometimes, our suspicions might protect us from harm. Sometimes our sympathies are what harm us. I found this quiz online, which I think is both funny, to a degree, and pertinent. It shows how much, or how little, we can tell by just looking at a person: Programming Language Inventor or Serial Killer?
We can play on these assumptions while writing. When we establish a character, there are things that writers might do in order to sway the reader one way or another when thinking about them. If Mrs. Danvers was plump and had red hair (as opposed to skeletal and yellowing), would we have regarded her with as much dread? If Atticus Finch was a foul mouthed drinker, would we have trusted and respected him? Every author, when writing, has a way he or she feels a character should be read.