Step 1. Have a short story idea for a submission call. Two years ago. It has one POV character and is, loosely, a romance.
Step 2. Start to brainstorm it. Run it by an agent, just for funsies, because you already paid for a pitch session and have nothing else to talk about.
Step 3. Hear these fateful words: “It sounds like a novel.” (Cue ominous music.)
Step 4. Yeah, it does sound like a novel. This would be pretty great.
Step 5. But it needs two POV characters.
Step 6. And an antagonist, obviously, who’s pretty awful. But he doesn’t need a POV.
Step 7. Enter the very important sister with a very detailed backstory—enough for her own short story at least.
Step 8. That villain is getting pretty loud. Definitely need to do some scenes from his POV.
Step 9. Some first chapter contests, some workhops.
Step 10. One round of NaNoWriMo (but only 30k words, because you’re working on other stuff at the same time).
Step 11. More brainstorming, more outlining. Because finally you’re really seriously going to finish up this story idea. No more sidetracks.
Step 12. Wait, what? That new character idea would really be super intriguing. She totally deserves her own sequel. But for now, she’ll just show up as a hint at the very end. Right?
Step 13. Wrong. She’s totally a POV character. Maybe?
Step 14. But at least I don’t have to write the sister’s POV! (Yet.)
And that, my friends, is how a 7k-word story idea I had a couple years ago has blossomed into today’s NaNoWriMo project, projected to end up probably somewhere around 80-90k, with four POV characters.