Nate Piekos has a great piece at Blambot explaining the grammar and tradition of comic book lettering. It’s worth a look for any screenwriter considering writing for the paneled medium.
Comic book lettering has some grammatical and aesthetic traditions that are quite unique. What follows is a list that every letterer eventually commits to his/her own mental reference file. The majority of these points are established tradition, sprinkled with modern trends and a bit of my own opinion having lettered professionally for a few years now. The majority of these ideas have been established by Marvel and DC, but opinions vary from editor to editor, even within the same company.
Many of the examples, such as when to use ellipses verus dashes, have parallels in modern screenwriting. But as a former font nerd, I was surprised I never noticed the rule about crossbar I, or the existence of breath marks. They were always there, but when used properly, completely disappear.
(Thanks to Daring Fireball for the link.)