Isaac Asimov, at his “best” (we can critique his literary skills elsewhere) could sit down and pound out a novel as fast as he could type on his clunky electric typewriter. With the kind of throughput he had, he had to be pantsing it, but I’ve found no references either way (but at 5k/day every day, I can’t imagine that he had time to plan).
I pants it (write without a clear outline), depending on my characters to pull me through the process because I know who they are, and what they want, and where they’re going. Doesn’t work for everyone, but at the last Armadillo Con writing workshop there was a panel of published authors. They went down the line: six authors and an increasingly bemused moderator. “Pantser,” “pantser,” “pantser…” You get the drift. Last was most surprising: an author with dozens of published murder mystery novels to his 90+ year credit. “I don’t know who did it,” he said. “Sometimes not until it’s all revealed.”
What he did know was his dramatis personae. See above, character, motivations, etc. Given that start it’s possible to “run with it.”
D&D dungeon masters (DMs) do something very similar (except, of course, for humans manipulating the player characters). And the DM has to handle any and all non-player characters (NPCs) that appear in the game. This is very close to the kind of pantsing I know. And there are some awesome dungeons with frantic DMs trying to keep one plot twist ahead of the characters.
For about a week I couldn’t get more than 1k words on a page. Some days under 100. And it brought me up short, because I knew the main characters pretty well. After coming through and then removing several scenes because they were flat and lifeless, I went back to my characters. Like solving an electrical problem in a car (before computers did most of the heavy lifting), I went and looked at every character and their interaction with others.
The “NPC” ones: the (first) victim, the mysterious stranger, the new characters on the block: they were all mysteries to mo. Why were they doing what they did? Why did they care about a better-defined character, or their actions?
So I took a step back (sans computer) and doodled on one of my writing notebooks for several hours. What were their names? Why were they in the story? What were they trying to get out of it? A few paragraphs of backstory, a clear physical and psychological description of each, and I was back in the driver’s seat, as it were.
At least, the seat by the keyboard that kept the words flowing.