B is for Back story
Back story is an interesting concept. You both want it and don't want it. Also, when you want it, you have to be careful how you dole it out. See? Confusing, right? Okay, here is the deal: Back story is the part of your character's life that you don't spell out; you only allude to it throughout your novel. But, in snippets. Too much is too much and too little isn't enough. Let me explain.
Say your character is sad. Simple, I know, but stay with me. You don't say "Sarah was sad because her husband was late coming home from work again." You start by dropping a hint about her childhood. About her memories of her own father coming home late from work. And the eventual discovery of the affair. And how it nearly killed her mother. Etc, etc. etc. You mention how it makes her feel twelve years old again. Maybe you have her wonder aloud if her own daughter now feels as she did. Voila! Back story. In a few short sentences you dragging in some drama from the past and stirred it all up with some drama for the future and you didn't bore your reader because it happened so fast they never saw the set-up coming. Back story.
The biggest thing to remember with back story is to sprinkle it in the story the same way you season food. A little goes a long way. And never underestimate your reader. He/she is smart. They will figure out the subtle nuance of what you are saying as easily as the unspoken drama of what you are NOT saying. But, don't force-feed them. You will get far more respect from your readers if you let them figure it out for themselves.
Think of a conversation you might have with a person who is new to you. You don't blab all over them about your divorce, your incontinence, your father's cancer, your grandmother's dementia, and that itty bitty gambling problem you have that you are sure you've got licked. You let out teeny chunks of your life in the conversation when appropriate. Do the same thing in your story.