I was asked: As an author, what makes a good character?
I replied: Don't you have anything better to do than ask complicated questions?
I like characters. LOTS of characters - the Traveler of Ord saga alone has upwards of thirty named individuals with significant roles to play over the span of two books, with more to come. It's possible I just want to play god in the second dimension, but I am of the belief that a novel-length tale needs lots of Toms, Dicks, and Harrys (oh my!) running around.
Why? To tell the tale from their own perspective, of course. A good character tells you, the author, how to write for them. A group of good characters reduces you to the role of taking dictation. One way to tell if you're working with such a character is to set aside your opus and write something from the character's viewpoint. A journal entry perhaps. The topic isn't important - just write from the character's perspective.
Go ahead. I'll wait.
Now, what did you learn about your character? Are you absolutely certain they are who you thought they were, or who you created them to be? Don't despair if you find your hero's armor doesn't shine one hundred percent of the time, or your capricious sidekick has some moments of melancholy. After all, can any of us "real" people apply a single line of thinking to every situation we come across? Then why should you expect your characters to?
I submit that the greatest characters - the ones that you will love to write for, are the ones who break out of the mold you created for them and adamantly tell you who they are. Even if it wasn't what you were expecting or hoping for. Even if they throw a sharpened ballpoint into the middle of your well-laid plans and force you to mold your masterpiece around them in ways you weren't expecting. You were so sure the butler did it, but...he's just too nice of a guy. Curses! Time to shift the blame to the haberdasher on the sixth floor that tuned out to be a much shadier figure than you were prepared to give him credit for.
The other great part is that now you have a handy-dandy little short story for your character that you can turn into a compilation after you repeat the process with the rest of your cast.