Backstory's a weird one. I mean, the usual advice is that it gives you the 'why' for a character's actions, and yeah, that's true. But I've read plenty where the backstory feels slapped on like a bumper sticker—'traumatized by fire' so now they're afraid of candles, that sort of thing. It becomes a cheap shortcut for motivation instead of the real, messy engine.
The trick, I think, is treating it like archaeology, not exposition. You're not dumping a whole tragic origin story in chapter two. You're letting the reader uncover it through habits, through the way they flinch at a specific tone of voice, through the seemingly irrational thing they cherish. That's where the motivation gets organic. Their present-day goal isn't just 'revenge for dead parents'; it's 'I will never be powerless like I was in that moment,' which could drive them to seek political power, or magical mastery, or just build a really, really high wall.
I messed around with a character once whose backstory was just a single, quiet humiliation from childhood—not dramatic, just deeply felt. Their entire adult personality was built around never being laughed at again, which made them ruthlessly competent but also painfully lonely. The motivation wasn't in the event itself, but in the psychological armor they forged from it. Readers picked up on the armor long before they ever knew about the childhood incident.