KTC's ideas are awesome, definitely worth trying out. To that I can only add:
be your character. No matter how different from you he/she is, get inside their heads, until you understand them completely and you feel what they feel, until every bad thing they do hurts
you and every good thing elates you.
Also, give them flaws (and good qualities to villains) to make them human. Make them understandable, logical in their action, but also interesting. (Which means you can write about illogical people as long as you understand them and make that understanding come across to the reader.)
Another important thing to make secondary chars seem real:
every character thinks he's the hero in his own story. Don't put every minor character's goal and backstory into the book (you'd end up with a thousand subplots), but
know them. That will show in your writing. Every character wants something and has his own life and plot. Don't rely on archetypes - when you use them, give them a story, and the archetype will turn human. And don't rely on a quirk or a single trait to define your character - explore them, make them complex.
Change is more difficult though. I had that problem too. It should be inherent to your plot; the story is about characters changing, at least in a character-driven story. Maybe yours is plot-driven? Anyway, an idea would be just to let the characters react. When something happens, don't make them do things, but try to listen to them and see what they'd really do. Put yourself in their shoes and see how those situations would change you. The plot should be reflected in the characters' inner evolution. It's hard to tell when I know nothing of your story, but getting to know your characters and their motives should help.
Of course, you might have already done half the things I said, but there you go.
