My big writing epiphany came while I was reading an old webcomic called Fallen. It stopped updating years ago and has disappeared since then AFAIK, but it will always have a special place in my heart for what it has taught me.
The main character was a woman with a pet snake. She discovers an amnesiac/ semi-amnesiac man who turns out to be a fallen angel, and ends up living with him. The big question of the plot was: what has he done to become a fallen angel? Was it some kind of a crime? Why did he do it, whatever it was? What was he like before the fall from disgrace (you get the sense that the quiet, mostly-out-of-it person you see is just him recovering from some sort of a trauma, perhaps physical, perhaps emotional, or possibly both)? In short...
Who is he?
"Who." Not "what." I'd read a ton of manga where the central plot twist revolves around "WHAT is this character?" e.g. Is this character actually the descendant of the hero!? Is he the villain's other self? Is she actually a weapon of mass destruction?
Most of the time, they'd try to add some "who" to the "what" by having the characters respond to the revelation. But the characterization is often weak, and ends up getting completely overshadowed by the "what".
My own stories were like that. I didn't even realize that was a problem until the big moment of epiphany. I think I went up like 30 levels in writing (from level -25 to 5, hahaha) within the span of 30 minutes that day...
@Menbung, are you Korean by any chance? Just a wild guess based on your username. ^^