In books and movies, it’s rare to know the entire history of a character before the actual plot begins. In fact, in many books and movies, the only backstory you ever get occurs in flashbacks, after you’re familiar with the character on a more pedestrian level.
It’s all well and good to expect the Players to create the important aspects of their own characters’ pasts, but it’s entirely another thing to put them on the spot, forcing them to marry themselves to a character concept they haven’t even spent any time playing yet.
In a game with Progressive Character Generation, or PCG, the Players “wear” their characters for a while before determining a lot of details about their history, psychology or values. In the early days of a character’s career it will be easier and more fitting to produce this sort of content, but no one follows a perfectly straight line in life, and everyone is multi-faceted. Learning a whole new angle on someone? Happens all the time.
Traditional fiction writers are able to go back and forth while writing, adding backstory and exposition to early chapters later, as it occurs to them. GMs should try to make their Players' jobs less difficult than that of the professional writer, not more difficult. This is why my approach to running "DayTrippers" includes PCG as a play technique, and if a Character Development Scene (flashback or whatever) is used to connect a character's background to their current situation, I reward it mechanically with XP.
The PCG approach gives the Player time to think about their character and see them in action a bit before committing to details that may or may not turn out to be important or useful. Instead, the Player learns about their own character just as we do when we're reading a book or watching a movie - or when we're writing one. In addition, if rewarded mechanically, it gives the Player an opportunity in every session to link their character's backstory to the current plot, as commonly seen in well-written stories and filmed entertainments.
Let's talk about all that.