Once you deliver that commanding first sentence or paragraph, you still have the challenge of keeping your readers’ interest. You can’t let them down, either by ignoring that lead-in or by totally reversing the tone the beginning has invoked. Neither can you distract them from the current action by breaking into a history lesson – also known as the dreaded exposition.
Many writers, myself included, make the mistake early in our writing careers of trying to squeeze entire decades of a character’s history into a few paragraphs all at once. That can get very boring for readers. We’ve taken away many surprises they could have discovered on their own, one surprise at a time.
You should definitely know your backstories for all main characters, good guys and bad guys. How else can you build even the vaguest of plots? Exactly what you do describe on those next pages depends, in large part, on what your book is about, as well as the tone you have already set.
Crime fiction may require building suspicion or doubt about the protagonist; or just plain describing the murder victim – again, not with the entire backstory in one basket. Action adventure may call for some tricky encounter with the bad guys or bad creatures. It may call for a description of packing a survival kit.
If you have set an ominous tone, stick to menace or threat. If your opening began with humor, weave that humor into your text – whether that is cynical, slapstick, or dark humor.
Let most of the backstory reveal itself as a result of character interactions in those first few chapters. Spread the backstory over actions, dialogs, introspection. And that is not easy.
I don’t pretend to be an expert – I’m still struggling to master the art. But here are some resources that may help you out.