Afternoon sessions at WIFYR were incredible and J Scott Savage's was no exception. It was well received in my workshop group.
Jeff, Scott, whatever - he goes by either, looks at what drives a character. You have to know their history to understand their motivations. Should a life-altering event occur that precipitates the story, their motivations change. Seems quite simple, yet it's good to be reminded of it. The writer must know where their MC is coming from, where they're going to, and how they'll get there. He says that is true of all the characters, though the minor ones to a lesser degree.
The word at this WIFYR was antagonist. (One of the words - the other was torture your MC.) Ammi-Joan Paquette and Jennifer Nielsen expressed the importance of a good antagonist. Your antagonist is a person, too, with a history and a goal. They respond to events and sets a new course for you main guy or girl. It doesn't have to be a person. It could be the elements or something else, as in Gary Paulsen's Hatchet.
Jeff also mentioned that the complexity of motivations grows as your audience ages. In chapter books, the motivation is me, me, me. Think Ramona the Pest. For the MG Percy Jackson, his concerns include me, but expand to family and friends. The stakes are the same yet raised in YA to more global, societal issues. The MC must prioritize their goals. One takes on more importance than the other motives and the character's actions will reflect that. Your early reader character has motivation for their actions. It's more complicated for the YA MC.
Think of various characters in literature. Who are they and what are they're motivations?