Motivational Interviewing. If you don't know what that is and you actually care, you should do some research. It's a good communication tool even if you don't work in a therapeutic field.
I love teaching. It gives me a chance to teach the way I'd want to be taught, and it also gives me a chance to watch how others learn. That tells me a lot--about myself, communication, and people. Today, it dawned on me I have been missing some opportunities in my writing.
One main skill of Motivational Interviewing is using open-ended questions. It doesn't do much good to guess at what is going on with a person giving you "yes" or "no" answers when all you have to do is phrase your question differently. You might get an answer you didn't expect.
Why have I never used this in consideration to my characters? (Well, yes, I do realize they're not real, but work with me here.) Usually, when I think through a hypothetical conversation with a character, it is more like, "You like the dashing hunk who just came on the scene, don't you?" Whereas, I suspect some of these very characters, if asked openly, "What do you want to do?" would say, "Eat some ice cream and take a nap."
Okay, this doesn't do much for plot, but it sure helps me know my characters better, and it can be a big hint if I'm getting off track trying to have them fit my mold. Fictional people are just as frustrating about not wanting to fit molds as real people. I promise.
Another important concept in MI is Stages of Change. Decisions don't flip on and off like a light switch and changes don't happen in one graceful swoop. It's messy, with jagged edges and changed minds. Maybe we don't need to see all that on the page, but you, the writer, has to understand it if you're really going to understand the psychology of your character. (Writers, please, if there's one thing you get right, let it be the psychology. Nothing spoils a book faster--I can handle a boring plot with a compelling character, but no amount of explosions and car chases will redeem a protagonist with the psyche of a sack of cat litter.)
If you want your characters to read as humans, they have to behave as humans, and that requires respecting them as such. If you think of them like real people and try a couple of these techniques, it might surprise you what life starts creeping into your work.