Back Story, Flashbacks, and Foreshadowing
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 backstory is boring. If you need the backstory to be part of the story, then start your story at the beginning of the “backstory.”
Think of backstory 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 THE GIRL ON THE TRAIN. It starts with Rachel on the train heading into London. There are hints about her personal problems and we’re told she has a fascination with a certain house, but we’re not told why. These are explained as the story unfolds. She’s divorced, an alcoholic, obsessed over what she’s lost and who took it from her. The story could’ve started with Rachel still married to Tom, suffering through infertility, his infidelity, but it would’ve taken away from the mystique created by creating knowledge gaps instead.
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.
In SUN & MOON, the story starts off with Katja in Dresden. The backstory of why she left Berlin is revealed gradually and becomes a big part of the third act. I could’ve started the story with the conflict that happened in Berlin and catapulted her to Dresden, but because this is a romance, doing so would’ve pushed the love story to the back burner, and changed it from a romance to a family saga.
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. Similarly, 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 to show him facing a fear early on, so when he did the heroic thing, it would be believable. To set this up I had him accomplish a scary test of bravery he had to perform for his Hitler Youth group.
In HUNGER GAMES we are shown Katniss's hunting skills with her bow before she volunteers for the games. Now we believe she can make all those great shots.
Foreshadowing helps the reader suspend disbelief when the twist happens.