There is in writing a term, "set piece," which is a self-contained passage–usually a scene, sometimes two or three. Such a module can be dropped into a book or screenplay here or there, or likewise, removed and replaced elsewhere, without greatly affecting the story. Generally, I use these to thicken character, or as an action sequence. The Twelfth Birthday is one I like: What was your character doing on his or her twelfth birthday? This can go a long way to establishing who they are, and I tend to show a pivotal event that starts them down the road they are on when the book begins.
Tooling along, and bap! a flashback, and there's Our Hero or Our Villain, on the day he/she (or as I've used it in my most recent novel, the term for somebody of indeterminate gender, "zhe") is having his/her ("hir") birthday and Something Important happens ...
Likewise, there will be an action sequence I know needs to happen–a sword hanging on the wall in the first act needs to come down and be unsheathed eventually; a build-up of the hero's and villain's antagonism needs a payoff; a new toy I have gotten needs a showcase. I know I will need one of these, but I'm not sure quite where, and better to write it when it pops up in my thoughts than not. Like waking up in the middle of the night with a great idea and being sure you'll remember it in the morning? Sometimes you won't. But if you write it down with enough tags to jog your memory, chances are better that you will.
If you are writing a scene in a white heat and it's going like gangbusters, don't stop to feed the dog, check your mail, or mow the lawn. You'll be sorry. When you get back to that sequence, it won't be the same.
One of my favorite set-pieces was in my first Star Wars novel. I wanted to have a scene wherein the droids, Threepio and Artoo flew the Millennium Falcon. I thought it would be hilarious.
My editors didn't want me to do it.
Can't have those two as VP characters, they said impedimently.
I can finesse that, I retorted confidently.
Naw, maybe not, they observed obstructively.
Please! I'll do it as set-piece. If it doesn't work for you, I can pull it out and replace it! I begged Heepishly.
Well. Okay. But we can tell you right now we aren't going to like it, they opined unconvincedly.
Came the phone call:
That piece with the droids flying the ship?
Yeah ... ?
Can you make it longer ... ?
Yes! I blurted fist-pumpedly ...
And of course, if they hadn't loved it, I wouldn't be telling you this story. (Although they wouldn't let me have the scene of the Falcon's crew at a gas station waiting to use the phone behind some alien guy talking to his girlfriend. In retrospect, I have to agree with that one. Too much.)
Um. Anyway, the point of all this: In my most recent novel for Ace, I had occasion along the way to do several set-pieces. I stuck these bits into a folder, and when I came to a place wherein it seemed there would be a space, I unshipped them, dusted 'em off, and shoe-horned them into the manuscript.
If you are lucky, you can do this without having to adjust the intro or outro at all. Sometimes, you have to rejigger a few lines, add or drop a players, whatever, because when you wrote it, you didn't know for sure who'd be there when you got to the scene. As you write, characters you thought you knew about at the beginning usually change along the way and become somebody else. And they had better change, or you are missing a bet.
Or a character might get killed in Chapter Eleven, which makes it hard for him to be there when the set-piece gets lit at the end of Chapter Seventeen ...
So I finished the novel, sent to off to Ace, my editor said, "Good work." and that was that.
Except that I managed to leave out one of the nicely-crafted set pieces I did. Just missed it, somehow. Couple-three thousand words worth.
It wasn't necessary for the plot, though it did advance the characterization of a couple of the players. Probably most readers won't miss it. I only found out when I went back into the ms looking for a reference and couldn't find it.
Being that one of the first things you learn as a writer is to never throw anything away because you never know when it might come in handy, I still have the scene. And since I am writing two more books using the same characters? That piece will show up in the next novel for Ace. And it's a day's work I won't have to do.
Never throw anything you write away. Because you never know when it will come in handy ...