The answer is obscenely simple, instead of fitting a character to a story we make the story fit the character. So how do we put the characters in that position?
For this, we have agency.
Agency is an action or intervention producing a particular effect. Agency is usually talked about in the context of games, but you can certainly apply it to a character instead. By presenting the character with some kind of conflict, we’re forcing that character to make a decision, and that’s where the magic happens.
Whether you’re writing a story or playing Dungeons & Dragons, you’ve already provided the situation with an overarching conflict. If you don’t a conflict in mind, why are you writing a story? And in the context of gaming, you already have a much more dynamic source of conflict, the characters.
William Faulkner once said that the human heart in conflict with itself makes good writing, because it’s the only thing worth writing about. We need to call our characters, or players, into action against this root conflict. In either situation each character possesses their own free will with which to react. This is a beautiful thing to work with. Whether the character at hand is a noble’s daughter fleeing an arranged marriage, a noble paladin tasked with protecting a lord, or a downtrodden warrior escaping their obligations, what happens when daddy comes to take you home, the person you are tasked to serve harms innocents, or your commander hunts you down and confronts you?
The Call to Adventure is often described as the hero’s acceptance of their place in the story. I don’t think that’s true. I put it to you that John McClain didn’t start offing terrorists because he accepted his role as the hero in the story, but rather because if he did nothing he would be unable to live with the consequence of dead cops, dead hostages, or (ironically) dead him. The hero doesn’t decide to act. They’re forced to act.
Whatever your overarching narrative idea is, that’s not really the story. Rather, the planned narrative proceeds in the background to some extent while the characters are facing their internal conflicts. On occasion the paths of your characters and you narrative idea will cross as they progress towards convergence, but your story itself is in the hearts of your characters and the decisions they make. So the question you should really be asking is “How do I force a person to do something?”