There are two wrong reactions to a rejection slip: deciding it's a final judgment on your story and/or talent, and deciding it's no judgment on your story and/or talent.

Fiction is about stuff that's screwed up.

What characters do must grow out of who they are, and who they are is, in turn, influenced by what you make happen to them.

Changers are characters who alter in significant ways as a result of the events of your story. They learn something or grow into better or worse people, but by the end of the story they are not the same personalities they were in the beginning. Their change, in its various stages, is called the story's emotional arc.

Characterization is not divorced from plot, not a coat of paint you slap on after the structure of events is already built. Rather characterization is inseparable from plot.

Technology is Darwinian. It spreads. It evolves. It adapts. The most dangerous wipes out the less fit.

You must learn to be three people at once: writer, character, and reader.