Build on what's established

Start with what you already know about this NPC and extrapolate. If the NPC is written up as a follower, monster, or threat, then their concept is already well-established and you just need to fill in some details. But if you're making up this NPC from scratch, you'll need to do more work.

No matter how sparse the details, you always know something about this NPC. How or why were they introduced? What can you infer about them based on that context? What have you already established about the world that can fill in the blanks? Go with the obvious answers or intentionally choose something that undermines a trope.

Hold your ideas lightly for now. As you ask questions and choose traits for this NPC, your initial concept for them might change. That's great! Stay loose!

I just introduced Elios as a good person to take charge of the defenses while the PCs head to Gordin's Delve. I figure Elios is an established farmer, maybe around Rhianna's age. I instinctively thought of him as a "he." I tell this to Rhianna. I'm now thinking of a good, well-liked family man, but we'll see what my players have to say.