This NaNoWriMo comment is cross-posted, and slightly edited, because I just realized it might be useful here.
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Here’s something I’ve never seen mentioned in any discussion of pantsing vs outlining (or at least planning). From both my personal experience and reading the screams of pain that pop up on the forums after NaNo, I think the need for some kind of outline increases with the complexity (and length) of the story. It’s probably the number one reason so many NaNoers wind up with a complete mess of a book that’s a nightmare to revise.
I have two very long stories — 12,000 and 16,000 words — written with no outline and very little planning. But they only had one plot line and a linear chronology. A novella that’s up to 43,000 words was written the same way, but it’s now reached the point where I’m having to make notes about the rest.
This year’s NaNo novel has several plot threads and characters, and moves back and forth in time. There’s no way I could write it without advance planning, if only to get the scenes in order. As I develop scenes, I find that some of them need to be somewhere else than where I originally had them. Shifting them around within the outline means that I won’t forget crucial scenes, and won’t have a lot of plot holes and continuity problems to fix later.
By the way, the other thing that’s seldom mentioned is your intentions for the finished novel. If you’re not planning to publish, none of this is really a problem. I’ve been steadily working toward cleaner first drafts because that reduces the amount of time I have to spend on revisions and edits to get a book publication-ready. It’s also why I do some light editing throughout November. NaNo isn’t really about fun, for me, except on the forums. It’s a month of hard work, following several months of planning, and that’s the way I treat it. If you’re doing it for fun, or for getting your fiction feet wet, anything goes. And don’t let anyone tell you different.