This is a nice topic to contemplate.
My current series, which is published here, Eigria: Lume actually has the character living in my head for real. Their backstory are easiest to get the grasp, while still being deep. Still, slight changes are made due to refining draft, but the overall concept are there since I started write the very first draft.
I consider the backstory, especially for those who is important in the story, from their family background. If they have no real family, how did they grow up? In what kind of environment, with what kind of people? What shaped them until the present moments of the story? What turned them into they are in the story? It came naturally as the concepting goes.
The only two I struggle is the twinsies granny, Benette and Benetta. Until today, I haven't found out, nor able to get info from them (kidding, but hey, they live in my head lol), how did they end up in there, yet they are very important to the story. I really have no idea about their backstory or how far I should go for them.
As for my other drafts, I dwelled too much into their backstory until I got lost at the present. I love the backstories, most of them not really made impact in the time of the story. So this is the struggle. It should have been the most impact, yet they are scattered everywhere, and they are not even puzzle pieces.