I sometimes envy authors who plan out not only their novel but a whole series of novels and know exactly what will happen in each book. I love the serendipity of working on a novel that surprises and delights even me (us) as it changes and grows, but sometimes it would be nice to know in advance how the things I write in book one will impact book three.
Say something in book one and want to change it in book three? There’s nothing you can do, because it’s there, in writing, and your readers know it as a fact.
One common subject that trips serial authors up is family. If it’s convenient in book one to have a character who is alone in the galaxy because their whole family was killed in a mining accident on Ceres VII prior to book one starting, then you don’t normally have that family turn up in book three at that character’s graduation ceremony. You might be able to wangle it by casting doubt on the accident—especially if your protagonist wasn’t there to witness the accident but just heard about it—and if you are clever in books two and three.
What you can’t manage so well is the number of siblings. If a character says they have no siblings in book one, and a sister turns up in book three, she’d better be a surprise.
Some authors seem to get away with it. In a recent book I read the protagonist’s love interest said, “No, I’m not married,” in book one. In book two it turns out he’s been married for ten years, and still goes back to see his wife (even if he doesn’t sleep with her). Since he’d already claimed he wasn’t married that meant he had lied to the protagonist. Which, based on the type of character he was, was unlikely.
It changed my opinion of the character, and I spent the whole second book trying to get over that fact. I don’t think the author meant to do that.
Sometimes, too, working your way around an inconvenience you wrote in an earlier story can make a future story stronger.
For example, there’s a whole new sub-plot for our only-child character mentioned above discovering he has a sibling he didn’t know about. Where did the sibling come from? Why didn’t our protagonist’s parents tell him about her? Is she really his sibling or just trying to get something from him?
Yes, sometimes, these little inconveniences you’ve written in aren’t inconveniences at all.