On the other hand, if you believe that you know your character well enough to get into his/her mind and tell his/her story, then do it. Start without that back story.

If, at some point in the telling, you realize you don’t know them as well as you thought, you can always stop and do the work then. It’ll work because, if you are this kind of writer — the kind whose subconscious knows your characters even better than you consciously realize — then you’ll be eliminating a lot of unnecessary work. If not, you can still stop writing long enough to give them a past.

It’s what I do and it’s worked for me for over thirty years. Occasionally I must stop mid-story and think about my character’s past. Usually, though, it’s not necessary and I’m glad I didn’t waste any time on unnecessary stuff.