This is doubly true for action sequences. I can't just know who the players are, who fights or runs away, who wins, who loses, who dies and who is captured. If I try and write the scene with only that laid out, I'm going to freeze up. Before I can really write the scene, I need to work it out, beat for beat.
I think this is partly because I approach action scenes, as a writer, from three different angles, all of which are detail oriented.
- As an RPG Player. I played a lot of D&D and GURPS back in my youth, and my group was totally that group, that kept adding more and more of the optional rules to make combat more complex. I never wanted those parts of the game to be just a collection of dice rolls and stats. I was always of the mind frame that clever choices, playing with the terrain. A dynamic trick or atypical use of an object was a winner in my book.
- As a film student. If you have any familiarity with the process of filmmaking, you know it is a meticulous process, at least if you want to do it well. Each shot has to be planned, practiced and set up before it's shot, and then edited together into something clean and exciting. Even with a gifted fighter like Gina Carano, you can't just turn the camera on and let them do their thing.
- As a stage actor. In my time on stage, I was in more than my share of stage fights. I actually had a degree of infamy for being able to take a hit and fall to the ground. But that was never out of control-- every move was explicitly laid out, step-by-step. Each performance would be prefaced with a fight rehearsal-- first at half-speed, then three-quarters speed, finally at performance speed. Get sloppy with that, someone goes to the E/R before the end of the night. This also gave me a certain amount of practical experience in what one can actually do with their body*, what it feels like to hold a weapon in your hand, the jar on your arm when you block a blow. Is it true fighting experience? No, but it's controlled experience, made to look as good as possible.
Having done that, now I know what I need, in terms of terrain, and in terms of who does exactly what, and when. Which made achieving the end-result I needed, storywise, much easier. Now that i have that plan in hand, the actual writing of the sequence is coming together very well.
*- The director I had for one show prefaced our fight-training, telling us, "Don't tell me, 'I can't do that.' This is your instrument." He pointed to his body. "When you're on stage, you need to know how to play it, or I can't use you."