Here's a little trick to get your players to either reveal or develop their backstories:
Just before you end a session, have an enemy call out one of your players by their character's actual name.
Not "you there" or "the big one." Their name. The one sitting in their backstory that nobody at the table has thought about in three sessions.
It costs you five minutes of prep. You pick one character, find one detail from their background, and put it in the mouth of the first threat they meet. A bandit captain who knows what they did. A guard who uses a surname like it's a warning. A creature that shouldn't know them at all.
What happens is your players immediately stop being passengers. The one whose name was called wants to know how. The rest want to know why.
You don't need to have the answer ready. You ask your player why they know their characters name and what they want. Your players will hand you the content for your next 3 sessions.
Works really well in both shaping your campaign and getting your players integrated and invested!
That's it. That's the whole trick.