on Writers Helping Writers:

A character’s backstory is where all the gold is buried in terms of their motivations and misbeliefs. Who we are in the present depends to a large extent on what has happened to us in the past. It’s critical for you to know your protagonist’s personal history—but that’s a pretty easy thing to work out. There are numerous character questionnaires available on the internet that you can spend hours filling out.

What you do with that information, however, is trickier—and everything depends on it. If you front load your novel with your protagonist’s life story or give them a variety of quirks and habits that don’t get used in any meaningful way, you will be squandering all that valuable information you amassed and possibly even turning readers away.