I’m going to say something you probably haven’t heard in writing classes before. Backstory is absolutely necessary to your book. But if done badly, it can ruin your book and keep your readers from ever reading past the first few pages.
What is backstory?
Backstory is everything that has happened to your characters before they appear in your book. You need to know this for every POV character. In particular, you need to know what their deepest wound is, how it happened, what events have reinforced that worldview, and how they are functioning in the world to protect that wound. You’ll also want to know what their happiest moment has been, because there will be a drive for them to recreate that moment, or a complete sense of hopelessness that it can never be recreated.
The tipping point
Up until your character appears on page, she has been living her life protecting her wound in a certain way. But within a few pages of your book opening, something happens that makes it impossible for her to continue living life the way she has been. This is often known as the inciting incident. She’ll spend most of the story fighting against this change. After all, life had been going along okay so far. Not great, but good enough. But we the reader know that if she’ll just give in to the change, she’ll be able to heal her wound and possibly reach that happiest moment she longs for.
All of that comes from the backstory. Think about when you get to know a person. You don’t dump your whole life story on them. In fact, some of the most painful events of your life take a long time to come out (usually the second half of Act II). But when you are living fully in your character’s skin, that uneasiness they feel, that protectiveness of their wound will all come out in the words they say, the choices they make, the things they do.
How to do it right
From the beginning pages, there should be a sense that all is not as it should be. That your character’s way of handling things might be fraying around the edges. Or something new happens that her old way of handling things just doesn’t allow for. We as the reader don’t have to know why this is happening. Figuring that out will keep us turning the pages. But you absolutely must know the emotional basis for why she does what she does. Everything that occurs in the plot has to tie back to that emotional basis.