It’s tough managing just three people in one place in a novel. When you have four, five, or a dozen, and there is action, I find it impossible to make the resulting prose plausible if I just plan it in my head. It’s too many things for me to hold and consider at once, without forgetting about characters, conditions, etc.
So what do I do?
It’s time to get out the action figures and plush. I set them up, assign parts and act the whole thing out. It becomes way easier to develop a plausible through-line for the action, keep everything in mind, and make sure that things “work.”
I just finished writing a huge action sequence in Chapter 7 that included 9-10 major players, and a bunch of other people too. Here’s a pic of the planning process!