Hello, folks! I've learned some of these things the hard way or picked them up here and there, and they helped me a lot while attempting to fill out a 50+ point in-depth character sheet.

Is it hard to enunciate the flaws of your character? If so, consider that your characters' greatest flaws may extend quite naturally from their greatest strengths. A character who is gung-ho about helping people regardless of the cost may also, thanks to that passion, be short-sighted and reckless, endangering himself and/or others in the process. He may also make decisions that seem reasonable in the short term but have long-term consequences he had not considered. If your main character is self-controlled and self-sufficient, maybe she rejects the opportunity to form beneficial bonds in order to remain independent, at the cost of her own goals. A pious person may be a rigid thinker. A fundamentally loyal person may not know when to let go if the object of that loyalty starts going down a bad path.

And one I'm still learning (though this is more of a plotting thing than a character-development thing):

Every once in a while, it doesn't hurt to let your characters, now armed with their rounded personalities, make mistakes and suffer the consequences and grow or change because of their decisions. It will make them better and more complex.

I fear that my tendency for diplomacy in real life makes my stories weaker. This month, I'm going to let my characters make big mistakes. (Hopefully.) It shouldn't be hard. My protagonist is a walking disaster.

Oh! Also. I love me some highly-detailed lists, but you can get a really, really in-depth look at your character's motivations by asking just three questions: What does he fear? What does he want? What will he do to get what he wants? We already got some of those questions in different terms recently, but it has helped me to consider them together.