You'd be surprised
From "Comics Grammar and Tradition":
There is no Em or En dash in comics. It's always a double dash and it's only used when a character's speech is interrupted. The double dash and the ellipsis are often mistakenly thought to be interchangeable. That's not the case in comics, even though it's rife in comic scripts. For the record, there are only TWO dashes in a double dash. It sounds like common sense, but you'd be surprised.
(I am a heavy em-dash user, as these are easily generated on Macs by hitting apple-option-hyphen; unfortunately, one of my e-mail accounts transforms my meticulous emdashery into question markery, making it look like I am casting doubt on every other statement!)
(Via Jenny D; I also realize that I am mimicking her style.)