A few weeks ago, while weeding in the overgrown plot alongside my yard, I yanked up a potato.
I found myself staring at the thing until I realized what it was, recognized the leaves, and I had an "Oh!" moment. Oh: last year, I must have missed one of the potatoes, and it had wintered over, and long after I'd forgotten it, the thing had done what potatoes do: it made a plant.
I stuck it back in the ground. But I realized our characters need their potatoes too. They need potatoes served three ways.
First, there's the character trait of which your character is barely aware. The reader is introduced to your character with this trait as a part of his whole, but over time readers begin to wonder why he's the way he is.
At some point, the plot yanks up something that looks like a weed, and hey, potato! It's the event or formative background that caused your character to become a liar or to hunger for justice or to fear a long-term relationship.
The reason my garden had potatoes was pure bad memory on my part: I moved something in the pantry and discovered a bag of sprouted potatoes. Given that I live in the Swamp, which as my Patient Husband says is "teeming with a surprising amount of life," I went into the overgrown leaf-dump and stuck the potatoes into the ground. "Have a good time, guys!" I said, figuring that at the very least, they'd turn into compost. Remember, I'm a New York City girl, and I know where vegetables come from: they come from Key Food.
The half-dozen potatoes surprised me by becoming four potato plants, which surprised me further by becoming twenty or thirty delicious potatoes last summer. I thought I'd gotten them all. Surprise!
Here's my second surprise: I'd forgotten the two that died, but later I discovered the two potatoes that never sprouted plants last year had put up plants this spring.
Here's your character's second potato: plot elements you introduced in the first half of the book should sprout in the second half. JK Rowling is an expert at this kind of thing: she'll slip something in, and then when it comes up again, instead of it being a new plot element, you say, "Oh!" Potato.
(Diana Wynne Jones is the master of this sleight-of-hand; the moment you realize what's going on in Archer's Goon is the simultaneous joy of feeling a story unfold and seeing how a black-belt level writer creates perfection. She does it just as well in Howl's Moving Castle. I'm told Beethoven is an expert at this too, but I'm not classically-trained enough to tell you where he does it, only that he does. Those amazing new musical elements in the development section actually arise out of the themes. Go figure.)
The key point you're aiming for is recognition. When something surprising happens, the reader should be surprised not because it's a shock, but rather because it feels right only he hadn't made the connection until now. It should seem obvious to the reader in retrospect: of course this would have come back to haunt the main character; of course this is who the main character really is; of course that's why these events were happening. Chekov tells you that a gun in the first act has to be shot in the third, but the potato corollary to this law is that a gun shot in the third act has to have shown up in the first. Has to have shown up and then been left underground, wintering over, forgotten, and then when everything seems dead at the end of the winter, it opens its eyes, and up pokes its head.
At the moment, I have seven potato plants, two from the dormant potatoes and five leftovers I missed in last year's harvest. And that brings you to the third.
Your third potato is the sequel. A sequel must have elements that were planted in the previous novel, but they have to be planted subtly and deeply enough that your previous books aren't filled with clutter. You have to plant your potatoes and then forget them. Leave them to winter over, presumed dead underground until one day the reader opens the sequel. He'll find that potato and remember when you planted it. Planted, but never harvested.
Jane Lebak is the author of The Wrong Enemy. She has four kids, three cats, two books in print, and one husband. She lives in the Swamp and spends her time either writing books or crocheting inappropriate objects. At Seven Angels, Four Kids, One Family, she blogs about what happens when a distracted daydreamer and a gamer geek attempt to raise four kids. If you want to make her rich and famous, please contact the riveting Roseanne Wells of the Jennifer DeChiara Literary Agency.