A backstory needn't be either fully developed when you begin playing a new character, nor set in stone after play begins. You can add details to it, add entire episodes in the character's previous life that weren't mentioned before, and even revise bits to make it better fit your personal vision and what the character is currently doing during the game.
So yes, keep a copy handy as you play, and make notes in it as ideas come up while you're playing.
Commentary by memnarch (who has not seen the movie)
Ohhhh, so that's why Jim was fine with Han dying; he couldn't share the secret backstory otherwise! Secrets aren't secret if they're shared around. And knowing what I know, this is going to take waaaay more than 30 seconds to go through.
Man, has Jim been bringing that tome of paper to all of the group's games since Han was created all those years ago? This is going to be an incredible mess of a story. Maybe not a complete mess since Annie has been helping, but it's going to be more than just the astrogeophysics that Jim is good at.
So there's been the two special dice Pete has carried, the video equipment that Annie's brought and set up, and now Jim's giant backstory. That means there are two more episodes for Sally and Corey to bring something odd. Costumes and fancy props perhaps?
Finn: I’m so sorry, Rey. I can’t believe your father’s gone.
Allan: Yeah! Now that he’s finally dead, I can reveal Han’s extensive backstory that I worked out!
Rey: Well, this should take about 30 seconds.
Allan: Here it is.
Chewbacca: That’s Han’s backstory? I thought that was a copy of your thesis.
Finn: I helped him write it.
BB-8: Huh. I wondered why you kept flipping through it and making notes while we were playing.
Allan: I’m always improving it!
Rey: I bet there’s a lot of room for that.
Allan: Sure is! I printed it with big margins.