Take a training class, learn a few things, piss off the instructor.
Education, my way.
In this case, the class was on developing character voice. Since I had already completed Finishing Kick and was well into Trail of Second Chances, it might seem odd to take the class after the fact. Wrong. I had a mountain that needed an attitude. My characters already had gobs of 'tude.
The class presented two different means for developing characters that were authentic: first, ask yourself what you would do if you were them, how would you feel?; and, have all the characters be facets of your own personality.
Remember, I'd already written one book and the main character was a teenage girl. A little hard to have that necessarily be a part of my own persona. And asking myself what I would do would have led to many more fistfights in the storyline. Instead, I tried something different.
I worked at getting so deep inside my characters heads (even the secondary characters) that if you pinched one of them, I'd say "ow!"
How does a writer do that? The same way a little kid becomes the hero of whatever drama they're acting out. In the case of one granddaughter, a princess. Who sings. A lot.
The grandson is a swashbuckling hero, saving the day, despite not having a clue on how to swash a buckle. He's also a champion race car driver and general daredevil.
Typical kid stuff - except for them, it's darn near as real as life. Imagination is a wonderfully powerful tool and children make full use of it. Adults, on the other hand, learned not to daydream, stay focused, get to work.
I've managed, at least since my late twenties, to find or create jobs that offered plenty of daydream time. It doesn't mean that it wasn't work - sometimes, it was darned hard work and none of it was in the creative sphere.
I spent a lot of years letting my imagination roam around, poking into nooks and crannies of my head. At the same time, I'm an inveterate people watcher. Do I want to go shopping at the mall with my wife? Not particularly, no - but I'll go to watch the people. See the expressions, the gestures, how they talk to each other and themselves.
People watching gives me the raw material. Imagination creates the golem, the inanimate human shape awaiting life. It takes one last ingredient to successfully climb all the way into a character's head, to bring him or her to life.
Thinking like a character only gets you to the start line. Until you feel what the character feels - pain, anger, mortification, joy - you're just putting your words in their mouths. Every story involves emotion whether it's a horror novel by Stephen King, sci-fi by Heinlein, or anything that Nora Roberts ever wrote.
It's the writer's job to articulate how the character feels at a deep and visceral level. Writers owe that to readers, to feel for the characters and with them.
My eldest daughter, who was one of my early readers, commented early on that I did a great job of channeling my 'inner girl'. One of the moms of my beta readers made a similar comment, saying that the dialogue sounded like her girls.
It wasn't by accident.