Having just offered a ton of unsolicited advice to an acquaintance who’s going from novels to comics, a lot of this stuff is on my mind.
Comics are not movies. Comics are made up of slices of moments rendered out on the page. This is not a hard thing to deal with, but it will take some time to wrap your head around it. Coming from scripting, it will not be your first nature.
Scripting will be full of directions like “Bob takes the still-throbbing brain of the zombie king and tosses it off the 44th floor where it plummets to the sidewalk below and splatters into a shower of twitching green goo.” This will not fly in comics. Not unless the artist you’re working with (and paying) has had a lot of training in comics and knows how to parse that out into a panel sequence that makes some degree of sense.
Film and video gives the writer/director/editor control over the flow of time. This is great in horror movies where the buildup is better than the release. You will not have this power in comics. You can have a reveal on a left hand page for a shock, but that’s about all you’re going to get. The reader in a comic can be active, flipping around in time and on the page instantly (since the page is simply a collection of slices of time). They will have control that you will not.
Don’t be afraid of written words. You have them as a tool. Please use them. Yes, this is anathema to film and video scriptwriting. You’re not making a film. You’re writing a comic. Even if you’re just writing a comic to option a film, you’re still writing a comic. If it doesn’t make sense, then nobody will ever buy your special idea. Write for the format that you’re presenting in. You can fill it with pseudo-filmic goodness, but it will never move. Your elaborate fight scene in the foundry while using improvised weapons that will look so cooool once you get today’s John Woo directing it, yeah, that has to make sense on the comics page first.
If you don’t pay your artist, you’ll never get your script to comic finished. Artists are not cheap. Good ones doubly so. And no, they won’t work for half of the back end. The back end doesn’t pay the rent or buy bristol board. They will not be your art monkey. And you’re not in a position where you can dictate to them. If they’re any good, please listen to them when they say “the fight in the foundry won’t work as you’ve written it.”
If your dialogue is bad, you won’t be able to hide it. If your story is weak, there’s no shadow to scurry to. It will not be acceptable just because “it’s a comic book, dude.” Bad work will still be bad, even if you’ve slaved over it. Don’t forget to be your own worst critic.
Honor the boundaries of the page. If you don’t, your comic will be unreadable. Err on the side of pages that are too light instead of too heavy, unless you are doing it consciously.
Long dialogues or monologues in comics will fail just as hard as they do in movies. If you have a page of unbroken speech, strongly reconsider your approach. Two characters talking in a coffeeshop is among the worst drawing assignments you can hand an artist.
Comics are not screenplays with pictures. They’re not just a storyboard, but you can try to aproach it that way (though it’ll show.)
Now, best of luck.