I find that when I 100% complete the story before I start drawing it out, I get impatient. There are certain scenes or characters that will take me years to get to. After a while I start to feel incompetent not being able to draw out my story fast enough. I get lost in the script, reading ahead to the parts that I can't wait to get to. I waste a lot of time doing this, and I would find myself doing this a lot. Eventually I would also get bored drawing, I'm more of a writer, not a drawer. This results with me starting up another story project to fill the 'need to just write' hole. Making it harder to work on the project I'm drawing because my creative focus is now on my new project. Most of the time I would eventually give up and stop working on the comic. I tried this method for several comic attempts, the one that got the furthest being Raven Wolf at around 250 drawing pages, which barely touches the script still waiting to be drawn out, my other attempts usually got around 30 - 60 pages in. (I still work on that comic actually, just very slowly) Writing comics was not working for me very in this way. I had to try something else.
I tried a type of 'create it as you go' method. Starting out with concepts, characters and a some character and story driven goals, and I left everything else open ended. It didn't work for me, it was too much uncertainty, and any new ideas that would change details or the world I could not use else I would have to re-do previous pages. It was frustrating, writing wise I felt trapped because I could not do a lot of things that I would usually do in a story creating process, I couldn't foreshadow, drop hits about characters or story, and I often I would not give myself the time I needed to flesh out events or dialogue, sometimes events felt pointless, and some directions would hit walls, and walls are harder to fix as you're drawing everything the same time you are writing it, because you cannot got back, or decide 'nope this isn't working, need to start over' like you can when writing a complete story. Also and most importantly, I felt like I had no goals, and ultimately, not point. I gave up pretty quickly on the comic I tried this method on only around 20 pages. The process was so frustrating that I never repeated the method.
The method that works for me was for me to have a cast of characters, a written and defined beginning, a written and defined end, and several key moments, events, stories that would happen throughout (in no particular order) some of these 'arcs' or scenes will not be finished and will be left for me to figure later. Essentially, the story has been written, but it hasn't been completely finished. It's still rough with a lot of cleaning up and polishing to do, with a lot of openings missing scenes and dialog. As I draw my pages I also polish out and finish the story. I try to have at least five or more cleaned up and finished chapters ahead of what I draw, and I never start drawing an incomplete chapter, but I never focus on writing so much that I finish going through the entire story. I need to be stimulated and active in both writing and drawing or I will lose interest drawing. So far this method has worked best for me, and my comic using this method has over 1000 pages and counting drawn to it now.
So yeah, a controlled, thought out yet still in progress method is what works best for me.