Passive characters are those who don't and can't be bothered to contribute anything important to the plot. The solution, give them something to do. No matter how small. Works for me.
Simple answer . Say you have a character to whom many things happen: They are abused, they are kidnapped, they are imprisoned. Say things like these are major plot points through the first half or so of the book. Many would say that this is a passive character, since he/she does not actually do anything significant untill a later part of a book. How do you keep the reader's faith that this character isn't a wallflower?
Depends. Does the character not do anything or does he/she try to do something, but it always amounts to nothing? I think a character who goes through all that and does absolutely nothing is totally unbelivable. Being a wallflower is perfectly fine. Being an idiot who lets the world walk all over you is not. I think you need to get more in touch with how "wallflowers" actually feel. Talk to one; I'm sure they'd appreciate it.
In response to the original post: A passive character is one who has no direct influence on the plot. Personally, I don't think they should be avoided. Not everyone is a man-of-action. The truth is, a lot of us would sit back and do little or nothing in most "fiction" situations. Passive characters usually lend realism in that sense. When they are finally moved to action, it's a momentous occasion and magnifies the event that spurred them on. For example, if "Bob the Busy-body" yells at the top of his lungs that the world is ending, we won't pay him much mind. If "Wally-the-Walflower" makes the same declaration, it is treated with much more seriousness.
Talk to a wallflower? I am one IRL. Anyway, maybe my use of the word threw off what I meant to say. Not shy, exactly, but... restrained? Impeded? Anyway, I'm mostly talking about a character who has no control of his or her situation. If the author is being true to life, then of course the character will try to do something about it; like you said, they won't just let the world walk over them. But in trying and failing and all attempts amounting to naught, will the character ellicit enough sympathy and empathy to keep reader affection, or will the reader think of them as useless? Heh. I'm confusing myself now. I suppose it depends on the character, doesn't it. And this also brings up a whole world of believability and reader involvement.
This is a compelling question. The feeling that the universe has us by the you-know-whats is a feeling I am sure most people can relate to at one point or another, but I don't think there would be much interest in a character who succumbs to this state unless it were somehow important to other characters in the story. As for someone who doesn't do anything in a story, like a set extra in a movie or a show, I avoid them at all costs. I will often introduce someone who seems like an extra, but comes into play later on in the story.
In my opinion, a passive character typically serves one of two purposes: (1) Inspires others to action by being the victim: Think of the brave knight saving the helpless princess; she does nothing and lends little to the story, but her plight spurs others onward. I'm a fan of characters like this, but don't make them be too helpless; the reader will no longer sympathize. And he/she must bring something to the table --something that makes them worth saving. (2) The guage by which all else is measured: It takes something extreme to stir a passive character into action. When that character is finally moved, it resonates powerfully and we realize that things have reached a climax. The mere fact that such an unlikely person is foced to act speaks volumes. It usually happens after all the "usual" heroes have tried and failed or when they simply aren't around. To your question about sympathy: If your character is earnestly fighting to get free of these troubles, the reader will sympathize. But if that struggle yields nothing, we'll eventually begin to feel that all hope is lost. Things can't go downhill forever; something has to come in and break that monotony. Whether the "knight" comes and rescues your "princess" or your character finds the knight within his/herself is really up to you, but something's gotta give.