Writing a comic script is an extremely regimented process. You’re often working within an extremely tight format that leaves little room for error.
John Ostrander explained it for us a while back:
First number: the number of pages. Right now, your monthly comic book is 22 pages long. Let’s say you’ve been asked to do a fill-in story or a complete in one story for a given book. There are certain space limitations you need to take into account.
How many panels are in a page? Well, your first page is usually the splash page which means one big panel. This page also usually has the title of the story and the credits box for the creators. Here’s some rules of thumb for the other pages: when there’s a lot of action, you use fewer panels per page. If it’s a talk scene, you can have more. I generally figure that it will average out to five panels a page. The splash page is one panel so you have 21 pages times five panels. We do the match and the whole thing totals 106 panels in which to tell your story.
That’s not a lot of room to work. And as we said earlier, every panel must convey an action. You have to be able to tell your entire chunk of story under those constraints, which means you’re going to have to make every shot count. Mark Waid explains:
In a 22-page comic, figuring an average of four to five panels a page and a couple of full-page shots, a writer has maybe a hundred panels at most to tell a story, so every panel he wastes conveying (a) something I already know, (b) something that’s a cute gag but does nothing to reveal plot or character, or (c) something I don’t need to know is a demonstration of lousy craft. Comics are expensive. Don’t make me resent the money I spend buying yours. Every single moment in your script must either move the story along or demonstrate something important about the characters—preferably both—and every panel that does neither is a sloppy waste of space.
The good news is that if you’re doing your own graphic novel, you can write to any length you need– but you still can’t waste any panels. So you have to figure out what actions tell your story, and that means that you need to make an outline… and that’s the next part.