That's a great question. I don't like to commit myself to anything in a character's backstory until I have to. I didn't know going into the West Wing that Bartlet had MS. Then, along came an episode where I needed to introduce the idea that the First Lady (Dr. Channing) was a medical doctor. And the way I did it was by giving Bartlet MS.
David Mamet have written some excellent essays on this subject. You can get lost in the weeds if you sit down and try to create an entire biography for your character. If this is what they were like when they were six years old, and this is what they did when they were seven years old, and they scraped their knee when they were eight years old. Your character, assuming your character is 50 years old, was never six years old, or seven years old or eight years old. Your character was born the moment the curtain goes up, the moment the movie begins, the moment the television show begins, and your character dies as soon as it's over. Your character only becomes seven years old when they say, "Well when I was seven years old, I fell in a well, and ever since then I've had terrible claustrophobia. Okay?
Characters and people aren't the same thing. They only look alike.
I write a lot of drafts of screenplays and plays. I keep writing and I keep writing; what I try to do at the beginning is just get to the end. Once I've gotten to the end, I know a lot more about the piece, and I'm able to go back to the beginning and touch stuff that never turned into anything, and highlight things that are going to become important later on. And I go back, and I keep doing that, and I keep doing that, and I'll retype the whole script, over and over again, just to make things sharper and sharper. That's for movies and plays. In television, there just isn't that kind of time. In television, I have to write a 55-minute movie every nine days, so we shoot my first draft.