Back story is the story that happened before your story starts. As the writer, it’s good to know the history of your characters--what brought them to the place they are now, and how those events have shaped them as characters.
But what you don’t want to do is give the reader all that information up front. This is an error that many beginning writers make. There’s a belief that if you don’t tell me everything about the character and what led up to “where they are now at the beginning of the story”, that somehow I, the reader, won’t “get it.”
A story that is front loaded with back story is boring. If you need the back story to be part of the story, then start your story at the beginning of the “back story.”
Think of back story like salt. You shake a bit on your story as you go along to add flavor. Too much in any given spot ruins the taste.
Let’s look at CLOCKWISE. I start the story when Casey is fifteen, watching a football practice at the school. Every thing before that is back story. How would the opening have worked if I’d spent the first chapter explaining when she started time traveling and how often it happens, and that she accidentally took her best friend back once, all before she jumped and caught the ball? Snoozeville. It’s all interesting information that needs to be told, but sprinkled throughout the story.
Or with TWILIGHT, Edward could’ve sat Bella down and explained the whole history of his life and the vampire clan in the first chapter, just so she’d really understand what she was getting into, but that would’ve made for a dull story.
Likewise with HARRY POTTER. If JK Rowling had started the first book with Voldemort killing his parents and leaving the scar on his forehead as a baby, that would’ve probably been interesting to read, but then we’d have ten years of his life to go through before being called to Hogwarts, and that wouldn’t have been so interesting.
Flashbacks should be avoided. Stories are interesting because they are happening to the character as we read it. Reading about something that has already happened, not so much. It takes a lot of skill as a writer to sustain tension and suspense while writing about something that has already happened (and obviously the character having the flashback is okay.)
Foreshadowing is an important writing tool. You’ve probably heard it said that if a gun shows up in chapter three, it better go off sometime before the end of the book. Likewise, if you want the reader to believe that your character would do something courageous at the climax of the book, you need to show him being courageous in the beginning. In my book PLAYING WITH MATCHES, I wanted one unlikely character to do something heroic that would cost him his life. He wasn’t the hero type but I knew I needed show him facing a fear, so I had him accomplish a scary test of bravery he had to perform for his Hitler Youth group.
In TWILIGHT, we hear about the bear attacks at the sport shop (Mike’s, I believe), before we know for sure it wasn’t the bears.
Foreshadowing is important to help the reader suspend disbelief when the twist happens.
Any questions about the above points? Let me know in the comments.