Once you've create a believable character (see my past entry on creating characters) with depth what comes next? As I go over my first two young adult novels I find myself thinking this over. Aside from just riding through the plot there are some very important things your characters must do through the course of the story.
When people experience things it often changes them, they learn and grow, or become bitter and introverted. Life changes people. Your characters should reflect this. Think of the horror novel where the woman always runs from the killer. It's annoying. The truth of the matter is she'll eventually learn to either stand and fight, hide, find a way out of the situation, or she will die.
Just like readers, agents and editors want to see character progression by the end of the novel and on an even greater scale by the end of a series. For example; in my young adult book my protagonist goes from being the shallow, popular girl to realizing the value of real friends and family and she learns to start accepting who she really is. It doesn't sound like a monumental change but in the course of a person's life, its pretty important.
If you want your book to get picked up make sure your characters grow and learn, or suffer the consequences! Check out this link to an agent's speech on why manuscripts get rejected: http://www.inkygirl.com/scbwi-2009-notes-reasons-why-your-manuscript-got-rejected/