Writing Characters, Not Plot Elements

So, you're deep into your writing process. Maybe you have a completed early draft of your story and there's just something off about some of the characters that you can't figure out. You figure out that some of them just seem like they're in the story because you want them to be, not because they really should be there. They do the things you want them to do, but maybe it's uninspiring and they just don't seem like real people when all is said and done. Why does this happen and how do you fix it? Here are some things to think about that might help elevate your character writing to the next level.