Jezebel wasn’t always a prostitute, Mary wasn’t always the mother of God, & Santa has a life beyond Christmas. https://sarahleastories.com/2015/12/29/writers-digest-wednesday-poetry-prompt-333-theme-exhaustion/
We all transform under the right (or wrong) conditions. Carbon turns to diamonds, water turns to ice, trees turn to paper. A transformative character is more interesting than one who is impervious to change.
Never overly describe a character. Include a few, pertinent details, then allow the readers to use their imaginations to fill in the rest, because nothing slows a story’s momentum than for readers to have to laboriously build a character in their mind according to the writer’s exact specifications.
It’s fine to write a fiction book with an agenda in mind, but never be more passionate about the agenda than the characters.
Authors are no longer limited to one character’s perception when they write from the first-person point-of-view. You can pull a “Picoult” (i.e. Jodi Picoult) with each chapter being told from a different person’s P.O.V. Just make sure the characters you use to tell the stories are equally compelling.
Draw up character profiles, even for short stories. A thoughtful reader will notice if one of your characters has blue eyes at the beginning of the story, & brown eyes at the end.
Every character has habits, or quirks, that make them memorable. The same goes for dialect & certain expressions they use. https://sarahleastories.com/2014/02/17/quirks-make-a-character/