Following PeeDee's advice, it also depends on how you are working with your artist.
If I'm running the gig, I like to get in with the artist before I even start properly writing the script. I know the general plot and the tone I wanna go with, so I chat with the artist and let him/her develop concepts. I MUCH prefer this way of working because EVERY time I do, they come up with little things, backgrounds, clothing, jewelry, etc., and it makes me think, "OK, what's that about?" Before long, the entire thing has become richer and more complex, and having those visuals, makes it easier for me to get into the script.
If I'm hack-for-hire, and I don't have the above luxury, I tend to be much more descriptive and detailed in the script because I want to make sure that the artist totally gets what I'm setting up.
The fractured nature of a script can be an issue, but the first thing I do is a set of 22 thumbnails for the book. I have a single sheet, template PDF file, (each page is the same dimensions as a standard US comicbook) that I print out. I then sit down and layout the pages and do little stick-men sketches for the book. Doing this I nail the flow of the book very quickly. From there I go and do the script. By breaking it down further, making it more disjointed, actually helps me sit down and flow through the actual writing part.
I never demand that an artist follow my layouts, but I know I'm not writing something impossible to fit in a page and I also get an immediate idea of whether I can fit a lot of dialog/bubbles on a page or not.
Anyone wants that PDF file can PM me. Might not work for you, but I couldn't do a script with out it.