The human mind seems hard-wired to judge others. It's not something we consciously do; it happens, whether we like it or not. First impressions are made in less than a second of meeting somebody. We even judge people before we ever meet them (also known as 'prejudice'). And it can be very difficult to change our judgements. But what is truly screwed up is not how quickly we judge people, but how we judge them.
We judge people by whatever we have from them. We often judge people based on their clothes and physical appearance. When we hear their voice, we judge them on it, as well as the words they say. We judge people based on their careers, even when we have no idea what their job actually entails, or how good they are on it.
And don't think that all judgements are bad. Somebody who looks attractive is judged to be positive or good. Somebody who works at a law firm is respected, whereas somebody working at a grocery store is looked down upon.
What is interesting about authors is that we are often judged as people by our text. If you are using your words to entertain, it could be great. Same thing if you are trying to change the world. But the funny thing is that some people can be amazing writers, but terrible human beings. The example people love to use is Orson Scott Card, who is lauded for writing Ender's Game, among many other works, and yet is vehemently against gay rights.
The funny thing, though, is that our opinions of people do change, and it is often when we find new information that shatters the assumptions we had of them. An author that makes you laugh, we assume, is the kind of person that is always happy and fun. But when you find out that they have violent anger problems, then we assume the author is basically always unhappy, but somehow uses the fun writing either as an escape, or as a lie. But if you actually meet the author and have a great time with a level-headed person, then we change our mind again and assume that the author is generally happy, with occasional bouts of anger.
No matter how deep we dig, no matter how much we uncover, we always judge based on assumptions. It is as true for authors as for our characters. And with regard to our characters, keep this in mind when you present them to people.
A good twist can be made by presenting a character in a certain light, and then exposing the reader to the bigger picture. A simple presentation creates assumptions of them, and when you find out that the character is more complex, it is a real shock. The reason that this is such a marvelous technique is that you never betray your character, so you create a legitimate surprise without any backlash.