For me, the biggest culprits for this are patterns of behavior that emerge because of actions and thoughts that 'feel right'. Eventually I realize that a character has, say, gone out picking medicinal herbs in the wilderness for the third scene, and that leads into "Oh. She's already interested in medicine, so maybe she wants to become an apothecary?". Or a pattern of feeling weak and lethargic may be indicative of a medical problem. Perhaps an existing backstory element may be expanded upon as I learn more about the character's mindset.

I feel like it doesn't necessarily have to be written, but those kinds of big realizations help me write characters like actual people and less like arbitrary collections of traits. I suspect most writers have feelings like this at some point or another.