Secondary characters. They can surprise you. I'd say that a secondary character, especially if you've placed them in a story for a specific purpose, need only be as deep as they need to be? Does that make sense? Yes, you want to avoid deus ex machina characters who only exist to help your heroes escape, or be that uber-secretive person in the background who becomes the heir to the throne in the last chapter. But the fact is, there will always be some characters that aren't as developed or fleshed out as others. You've got to give the spotlight to those who matter, and let the others just hold their own. You don't have to have a complete life history to every single face and name in the book. Sure, some people do that for fun, but sometimes I think in order to create a minor character, you only need to include a few specific details to plant them in the reader's mind, and then let the reader conjure up the rest on their own.
And if they fill an important role, but there doesn't seem to be any sympathetic conflict going on around them...create some! Ask yourself what would make you care about this person, then put it in there. I've often been surprised how characters that I first introduced as secondary became very important and full of conflict later on as more information was revealed.
