Scriptwriting doesn't just have to be about moving image. Comic book scripts aren't completely different from TV or film, but there are some things you need to bare in mind if you're moving from the screen to the page. This is a friendly guide, not strict laws. If you want to do something your own way that works, that's fine.
Setting the scene
When it comes to setting a scene, don't be as emotive in your description. Artists might like some guidance in where the action takes place, but they might also desire a measure of independence. Give the atmosphere ('clean', 'simplistic', 'tense'), who's in it and what they're doing. The artist will take care of the rest.
In terms of dialog, the general rule is 10 words a speech bubble, though allowances can been made. If it flows nicely, use long speeches. Panels normally consist of up to 7 a page, in order to manage how much the reader sees. The average page count (currently) is 24, including covers. This means you have to make the story concise, every page being important. Too long a book, and people lose interest. Too short, and they feel robbed. If you're writing a long story arc, this doesn't matter as much.
This is tricky, as a wandering eye might be able to see the trick up your sleeve if it's on the right hand page. To beat this, try and put it on the left-hand page, so as soon as the reader turns the page-BAM! They're caught by surprise. In the script however, don't keep secrets from the artist. If the old man is secretly Captain America, say that, so the artist can drop hints.
When it comes to character, the writer's job is to showcase just that-the character. Describe briefly what they look like, with a mention on costumes/outfits. Some artists will do it themselves based on the personality of the characters.