When you're developing your characters, what do you do to make them more believable? And how do you make them all unique? I feel like most of my characters are flat, or else are all very similar.

So characters…I can talk for a very long time on character development, because it’s just as important as plot and even, I might argue, more so.

The most basic piece of advice I can give is “Know Thy Characters.” Know them as well as you’d know a good friend. Of everything you know about a character, only about 10% should appear in any given book, meaning you know 10x more than you need to for the purpose of the story. That’s what makes them believable and unique.  They have a past, a present and a future, hopes and dreams and fears, and even if those are never mentioned outright in the story, they will shape your character and his/her actions.