Writing guide: foreshadowing

Purpose

The purpose of foreshadowing is usually a way of adding suspense right at the beginning of the story, and in such a way that's both subtle and foreboding of future events. How subtle you are will determine how big the impact of your foreshadowing will be on both the reader and the story overall.
The most subtle hints, those the vast majority of readers will only pick up on when reading the story for a second time, are usually mostly a clever way of hinting at future events without really setting the mood, and also a way of showing how much thought and planning went into the story.
The slightly less subtle hints, the ones most readers will pick up on, are used to set the mood, raise the stakes, and ultimately setup a contract of sorts with the reader. Foreshadowing will promise a certain event, the reader will get to read that event if they keep reading. So it's up to you to deliver upon this.

An example of the more subtle hint most people don't pick up on is the movie Shaun of the Dead. At the start Ed plans to take his friend, Shaun, on a pub crawl to help cheer him up. "Bloody Mary first thing, bite at The Kings Head, couple at The Little Princess. We'll stagger back here. Bang! Back at the bar for shots."
That line may seem like nothing more than a pub crawl plan, but it actually gives the entire plot of the movie. The first (bloody) encounter is a zombie called Mary, later a zombie bites Shaun's stepfather, who's also the king of his family, and they then save a couple and Shaun's princess. After that they literally stagger back to the pub, and it ends with gunfire (shots).

As for slightly less subtle hints, in The Empire Strikes Back, Luke Skywalker duals with a vision of Darth Vader in the Dark Side Cave. He manages to take down the vision, but he finds his own face behind the mask of Darth Vader, hinting at the later reveal that Darth Vader is Luke's father.

Foreshadowing has to be subtle though. If it's not, you end up ruining all the suspense, make it look cheap, and you'll essentially have wasted a great story telling opportunity. You can hint that something will happen, but you cannot give away what will happen. The point is to keep the readers guessing, to make them think about what could happen, and perhaps even make them look for more clues.

Relevant & required

You may be wondering whether you need foreshadowing in your story, and the answer is dependent on the type of story you're writing. Not all stories need foreshadowing, and the purpose of foreshadowing should help you answer the question of whether your story needs foreshadowing or not.
Foreshadowing is best planned during the planning phases of your novel anyway, so try to figure out if you can work it into the story in story relevant moments. Foreshadowing's also best used for the biggest, and most impactful events in a story, but not all events lend themselves to being foreshadowed in a smooth and natural way. Don't try to force foreshadowing into the story either, this'll only end up creating misleading and otherwise disruptive elements.

Foreshadowing also isn't the only way to build tension, hint at outcomes, and so on. You can build suspense throughout the story by raising the stakes, sometimes the outcome is hinted at simply be the type of story you tell (good usually wins over evil for example), and so on. So while foreshadowing is a powerful tool, it isn't an absolute must have in every story.

The plant

So, how do we actually go about adding foreshadowing in a book? Well, the first thing you'll need is a moment, sentence, or perhaps even just a word that will act as a seed. You plant this seed early on in the story, and ideally it'll grow in the subconscious of the reader until the reveal happens. If the hint was just subtle enough to not be an obvious give away, but memorable enough so the reader remembers it at the end, the reader will put one and one together when the big reveal happens.

The foreshadowing hints do not have to be memorable however, as I stated earlier, they can simply be clues people will only pick up on when they read the story a second time. Take Fight Club for example, there are hints scattered throughout the entire movie, even spliced frames of Tyler you'll never see on a first viewing, or even a second, third, and fourth viewing, but once you know about all the hints, the movie will become a whole different experience. Foreshadowing can do this, it can change the perception of the story as a whole, make it all seem like a carefully planned story, which ideally it is.

The pay-off

Of course, once something is planted, you will need a pay-off. Anton Chekhov, a renowned playwright, once said the following: "One must never place a loaded rifle on the stage if it isn't going to go off. It's wrong to make promises you don't mean to keep.". The plant is the gun in this case, and the pay-off is the firing of the gun.
Obviously foreshadowing hints are often way more subtle than seeing a loaded rifle on screen, so if they "don't go off" it isn't a huge oversight, but it can still come across as clumsy or as a forgotten element.

The pay-off shouldn't even be difficult to deliver either, as you usually create foreshadowing based on the pay-off, rather than the other way around. You'll know what the big events in the story will be, so creating hints based on those afterward are obviously easier than when you try to first create hints, and then figure out some kind of pay-off for those hints. One thing you will have to keep in mind is to keep the pay-off and the plants in sync. You don't want the hints to promise an incredibly dramatic pay-off if the pay-off won't be that dramatic, and so on.

On the other hand, you can fool the readers into thinking "the gun" will be used a specific way or by a specific character, but ultimately have it play out differently. Obviously you don't want to be misleading to the point of coming across as cheap or vindictive, but it can be a very effective way of both raising tensions and surprising the reader.

Types of plants

As mentioned earlier, I'll go over a couple of examples of how you can implement foreshadowing. These are by no means the only or best ways of using foreshadowing, as this depends a lot on your story. But hopefully they'll at least provide you with either inspiration or an illustration of how foreshadowing can work.

Anxiety

Anxiety, in all its forms, is always an easy way to make scenes more tense. People are anxious for a wide variety of reasons. They could be nervous for a test, waiting for their final train, hoping somebody doesn't find an item while their room is being searched, and so on. Anxiety, as well as our demeanor in general, shows a lot about what we think in a given moment, and can thus act as a big hint at what might be playing both in our minds and in that specific moment of our life. Here's a less than subtle example of how it could play out:

Jordan's knees trembled heavily. He tried to stop it, but it was no use. A man dressed in a dim gray suit opened the door leading to the courtroom, and signaled it was Jordan's turn.

This basic setup shows Jordan's nervous, so depending on what Jordan has to do in the courtroom it could indicate why Jordan's nervous. Perhaps he's guilty, perhaps he has to testify against a friend or a dangerous figure, perhaps he just really dislikes talking in front of an audience. Depending on the answer, you can add more elements, make the original way more subtle, and even skew the expectations of the reader one way or another. Perhaps he's nervously playing with a ring that later turns out to belong to the victim, perhaps he was calm until he received a mysterious phone call, or perhaps he's simply putting on a show. Each will make the story completely different, and each gives the reader different expectations. Remember that each hint is a promise you'll have to deliver on, and foreshadowing is supposed to be subtle.

Character opinions

Another method is making the character state their opinion about their life, an event, or something else relevant to the story overall. It could be a statement of determination ("I will find the killer."), a wish or hope ("Everything would be so much better if I found somebody to hold at night."), or something else entirely. When it comes to foreshadowing, character opinions can be trickier to keep subtle, but they're also easily forgotten by the reader, meaning the connection might not be apparent until a second reading.

Dialogue

Dialogue can be a very subtle way of providing foreshadowing, as spoken language often allows for more playful ways of conveying different messages. For example, "This job'll be the end of me." is a common phrase, and people obviously don't mean their job will literally end them one day, but in a story it could foreshadow that their job will indeed be the end of that character toward the end. There are plenty of phrases like that ("I'm so happy I could die.", "What would you do if you only had 1 year left to live?"), but they don't have to be common phrases. As the Shaun of the Dead example showed earlier, with some clever work you could hint at far deeper, funnier, or other creative events.

Irrationality

Humans are irrational in many ways, especially when it comes to fears. Playing in on these irrationalities allows for some creative foreshadowing, if used right. Because while irrationality is incredibly common in real life, in a novel the mere mention of an irrational fear will tell the reader something will happen. "Don't worry, I'll be fine." often means that character won't end up being fine.
Using irrationality subtly can be difficult, the previous "Don't worry" line's definitely on the nose for example, but there are always ways around that. You can use it to make the reader think something will play out a specific way too. "Don't worry, I'll be fine." could turn out to be true, but maybe the person who that line was said to turned out far from fine.

Prophecies

Prophecies are a common way of using foreshadowing, but they obviously don't work for all types of stories. They're not always the most subtle, but the tension comes from wondering whether prophecies are real in this story universe, and whether the character will be able to avoid it if desired.
Prophecies are somewhat similar to irrationality, as in real life prophecies are common (horoscopes, fortune cookies, hand readings, etc.), but they're often discarded by most people. In (good) novels everything's written with a purpose, so if a fortune cookie is mentioned, there's a reason for it. The reason could be something unrelated to the actual fortune, but that's where the tension comes into play.

Showing it

Showing a foreshadowing event can be a very subtle way of adding foreshadowing, and you can play around with the subtlety a lot too. A person walking into a bank after putting a gun in their pocket is definitely foreshadowing, but if you only show they put something in their pocket, rather than the actual gun, the surprise is bigger when a gun is pulled out eventually. You could go even further by only describing the character shifting the weight of their jacket a few times, indicating it's being weighed down by something, in this case a gun. But in this later case we have no clue where the weight is, it could be in the left or right pocket, inside the jacket's lining, in a pocket on the inside of the jacket, and so on.

(Pre-)scene

Sometimes a simple scene could do the trick. They're not always the most subtle either, but there's a lot of detail you could hide or omit. A Song of Ice and Fire starts with a pre-scene warning the reader of the White Walkers, and telling the readers that they are indeed real, something many characters in the story do not believe. It might take several more books before they become an actual threat to most characters, but the foreshadowing was there from the very beginning.

Scenes will definitely make the reader pay attention to the information within it. But within this scene you could use some of the other foreshadowing methods to add multiple layers of hints. Of course, the savvy reader will be looking out for them more in scenes like this, so you may wish to make them more subtle than normal.

Symbolism

Symbolism can make for subtle hints, but to some readers they could also come across as very on the nose. Something like mail not being delivered one day could be a subtle hint that regular life is ending, which is quite subtle. On the other hand, something like a fallen horseshoe wouldn't be very subtle.

You can play around with symbolism a lot too, or use similar elements to create a subtle hint. For example, castor oil and ricin (an incredibly deadly poison) come from the same plant, so the use of castor oil before a death or throughout the story could be a subtle hint that ricin was a murder weapon.
Obviously not everything will have a connection like this, and in many cases some research will be required. But research is always a good thing to do no matter what kind of novel you're writing, if nothing else, it might at least provide accuracy and some inspiration.

Learn from others

Speaking of research and learning, pay attention to those before you. Foreshadowing has been done countless ways by countless writers, so pay attention, and you'll pick up a trick or two along the way. This goes for everything of course, not just foreshadowing, but subtlety can be a tricky thing to get the hang of if you're not used to it, so read and learn.