If a man is going to shoot someone in Act III, he must acquire a gun in Act I.
You’ve probably heard that, or something similar to it, before. It’s an old maxim that explains foreshadowing. Things cannot happen in a story just because you want them to, or need them to. If Mary is going to shoot her husband, she cannot just pull a gun from her purse at the necessary moment. The reader needs to know where that gun came from and how it got in her purse. Events must spring logically from events that have come before them. And it’s your job as a writer to set up those events by preparing the reader now for things that will come later.
There are several types of foreshadowing.
Blatant: Here, you’re telling the reader straight out what your character will be doing in the future.
Her mind was made up. She would kill her husband tomorrow.
Implied: This makes clear what will happen in the future without coming right out and saying it. It implies an event through action or dialogue, but is still pretty obvious to the reader.
She picked up the photograph of her husband, and with marker in hand, drew a fat, black X through his face.
Subtle: This is less obvious. It’s slipped into the background carefully so that it’s barely noticed.
She hurried down the street to the bar where she’d have to pry her husband from a sticky bar stool yet again. She passed several strip clubs advertising Girls! Girls! Girls! and a pawn shop window filled with the belongings of others - rings, watches, a tv and pistol, two guitars. And then there were the porn shops . . . .
You can also work foreshadowing into your manuscript in several different ways.
"It’s going to be a tough game tonight. Do you think we’ll win?" Kevin asked.
"Of course we will," Tom said. "Jack’s back from visiting his Grandmother. We can’t lose with him on the team."
"Who are you asking to the dance?" Jake asked.
"I’m gonna ask Sue," Tom said.
Jake shook his head. "Bob already asked her. She’s going with him."
Tom smiled. "I think Bob’s going to be disappointed."
Let’s say your character has to swim out into the ocean to save someone from drowning. At some point before this event, you can add in a flashback to his days as a Navy Seal or swim coach to establish that he has the capabilities to swim the distance and rescue the drowning man. But don’t just use the flashback to establish he can swim. Make it do double duty. Perhaps we learn in the flashback that he once failed to save someone else from drowning, which is why he drinks now. Then when he swims out to save the drowning man, there will be no guarantee that he will succeed. You create tension and suspense.
You can use description to foreshadow an event that will happen in the same scene instead of a later one. Leonard Bishop, in Dare to be a Great Writer, compares it to the background music in a movie. If you’re just listening to a movie instead of watching it, you can still tell when something’s going to happen because of the change in the background music. In writing, instead of changing the music, you change the writing. Slowing it down and dwelling on the description will tip off readers to pay attention. Something is going to happen.
Mary slid out of the car with her groceries and headed for the house. She slipped her key in the lock and turned it. She stiffened. It was already unlocked. Had she forgotten to lock it?
She opened the door slowly and stepped inside. The curtains were closed and the room was gray and filled with shadows. She crept silently across the carpet and set her bag of groceries on the couch, listening for the slightest sound. Everything looked the way she had left it. Her purple sweats hung over the back of the sofa, an empty coffee cup sat on the coffee table beside the jigsaw puzzle she had been working on. She stepped carefully toward the kitchen and suddenly stopped. Looked back. The puzzle! There had been two pieces missing in the sky. Now there was only one.
And, of course, you would have set up this scene by foreshadowing it earlier before Mary ever went shopping. You would have shown her working on the puzzle, searching through the box for those two pieces of missing sky.
If you use it wisely, one bit of foreshadowing will lead to another, which will lead to another, which will do more than set up a future event. If done well, it creates a thread that strings your novel together, that creates tension and suspense, and that advances your plot. It might even foreshadow a book sale.