I hang out online with a bunch of folks, one of whom is Oliver Campbell (Rabbit in the Road, The Twisted World Verse One: The Dusk Harbinger). While in his Twitch channel the other night, he started talking about character and motivation. I wish I had recorded it.
It's hardly difficult to find books, articles, blogs, etc. on the importance of motivation in creating and conveying believable characters. It's something else to hear it taught the way he did, while playing The Binding of Issac: Rebirth, that other night.
He talked through exercises. Imagine a dude with asthma; how does that change how he things, acts, and what he worries about? This meant that you work your way into seeing things from that other individual's perspective- one that is NOT your own. Using that knowledge, you put words down that bring forth this character on the page and from the interaction of perspectives with circumstances you soon find narratives writing themselves. Once you know what motivates a character, figuring out how and why they do what they do is easy. That's how you get the narratives writing themselves, leaving only the details to has out. Knowing his perspective brings you into knowledge of that motivation.