Everyone knows how important first impressions are. Fair or not, the first impression you make tends to form the filter through which all your actions are judged. If the first impression you give someone is that you are lazy, then he tends to view every instance of your avoiding a task as an example of this laziness, rather than as evidence that they're overworked. This is the logical fallacy of confirmation bias, where we tend to put more weight on examples that confirm what we already think, than on those which contradict or mitigate it.
This is not just a problem with first impressions. You can meet someone and think they're perfectly fine, but if you're later told by a friend that he's sort of sleazy, then every awkward conversation and off-color joke becomes an example of sleaziness, not simple awkwardness.
This is the problem of labels. We tend to think of labeling as a problem of ethnic stereotypes and bigotry, but it's really an example of the fact that the human mind likes to sort and categorize people, and make decisions based on those categorizations. To some extent it's a helpful short-hand that makes up for the fact that for the majority of people we meet, we'll never know them well enough to really understand how they think. But the problem is that it's never that simple, and because of the problem of confirmation bias, we're actually not very good at telling when that label is wrong. We tend to weight facts and incidents that confirm our label more heavily than those facts which work against it. This can lead to very ugly prejudices in real life, where someone can't escape an unfair label.
For a writer, labels are both useful and dangerous. When you need a minor character, a quick label is a fast way to give a character a recognizable personality without needing a lot of detail. For minor enough characters, this may be enough. But if that's the entirety of your characterization for a recurring, or for that matter, a major character, then they can start to seem flat and static. As your readers get to know your characters, they should begin to see the cracks in the labels: the contradictions and complexities. Which is not all that different from when you start to get to know a real person better.