Honesty, character backgrounds change as I am writing. I come up with a general idea and write it all down so I don't forget. It happens and I have go back and check myself especially on their physicals appearances. Their appearances change more often than the backgrounds do, but I'll talk that more later.
When I come up with an story idea, I wrote out a general description. Male or female, approximate age, most of the time their profession, and then major thing in their past that makes them who they are. For example, are they divorced, widowed, or just terrible at relationships? Did they have a fairly happy childhood or were they abused? Things like that give me a concrete beginning. The most important part though is which characters have faith and how deep that faith is.
As I am writing, I change many things such as number of siblings or professions. Sometimes the profession plays a huge part in the story and others not so much. I contemplate professions probably more than I should, but I don't like repeating myself from story to story. Siblings usually get added as I am writing so that my story plays out the way I wish depending on the type of story I am writing. It helps when shape my hero/heroine. I have noticed that most of my characters usually have no more two siblings, but I am working on fixing that.
The involvement of parents varies much like the siblings. Sometimes a parent's death plays a huge role in the story.
I often find myself backtracking to add the main characters' friends or even pets (sometimes I forget I gave them a pet in the first and have to add them in randomly once the story is complete).
There's a lot going on in my brain when I am writing and sometimes those little details don't matter in the big picture. I often find those errors when I am proofing. Such as in one story, I gave the hero a dog in the first chapter and then completely forgot all the other times the hero was at home.
In my notes app with my character info in it, I put questions by anything that I see as changeable for a character or if I am unsure that is the direction I want to go. It's just an idea after all. I can be halfway through the plot and change to make it fit the direction I decide to go. Sometime I end up deleting a whole chapter because it did not play out the way I expected. Can't force it.
As far appearance, they are usually set in stone because I have a certain vision in mind. Sometimes I find myself typing the wrong color eyes or hair. I read over a completed story yesterday and realized the hero's hair and eyes were all wrong. It was no longer the picture I had of him, so of course I changed it. It's the good thing about it not being published yet. (Maybe one day.) I have thirteen novels/novellas written amd most of the time mutiple in progess so sometimes I get my character appearances mixed up.
That's what proofing is for. The characters also change as events play out within the story so it's natural for my vision to mold with it.

Questions? ;) To each his own!