As a kid, I loved reading comics, and wanted to be a comic strip artist much like Charles Schulz [1] or Jim Davis [2]. I had even attempted once to draw a comic book. What I don't recall is how I wrote the comic book.
I had seen how comic books are written, thanks to an oversized (nearly poster sized) Superman special comic book, and it looked more like a screenplay than a comic book (oddly enough, most movies, or at least those made by Messrs. Lucas and Spielberg, go through a storyboard phase which looks more like a comic book than a screenplay). So all these years, I kind of assumed that's how comic books are written.
Not really [3] (link via news from me [4]).
Or rather, it's really up to the writer, as this Porky Pig comic script written by Chase Craig [5], shows.
[1] http://www.schulzmuseum.org/
[2] http://pressroom.garfield.com/jim_bio/
[3] http://www.michaelbarrier.com/Essays/PorkyPigScript/PorkyPigScript.htm
[4] http://www.newsfromme.com/archives/2008_01_29.html#014723
[5] http://www.michaelbarrier.com/Essays/PorkyPigScript/PorkyPigScript1.htm