Great Characters are born not written

Developing great characters is the most difficult part of writing, in my mind, and is one of those things that both agents and publishers latch onto quickly.
It is inconsistent, though, and I have had agents say "Your characters are weak," and "At least the characters are really strong," all about the same characters!

So, ignoring agents entirely, here are my thoughts on not how you write about your characters, but how you find them in the first place.