Find their vulnerabilities and target them. Only if their fears are challenged will they find the strength to overcome them. The most successful authors are those that can pack a story with emotion. I'll watch it over and over. It all kind of mushrooms.
Some authors can sit down and create a story as they write.
Motivations have to make sense and seem viscerally real. Oh, and it has to be exciting. If I find it boring, forget it. Other times I use the outlining feature in Word. I decide on scenes that will advance the story.