What would your character do if approached by a drunk in a bar? Lost in the woods with no food or flashlight? Locked unjustly (or justly) in solitary confinement? If they were told they just swallowed poison?
This is an exercise I was just doing recently with my sister, and it’s fun and also informative. Trying to imagine your character in different situations (even if completely unrelated to the story, or even impossible for the world the character lives in) can help you get a clear idea of who they are, or let you know that you don’t really have an idea who they are at all.
Try it out, you might learn something about your protagonist!