K. Parille takes a look at Charles Schulz’s punctuation in Peanuts:

His work is only one example of the ways that text in comics — and especially in word balloons — is liberated from the kinds of ‘rules’ that govern prose. It’s a way that comics can be aligned with poetry, which shows far more openness and freedom with punctuation. Schulz, for example, almost never ends sentences with a period, a standard stop in essays, short stories, and novels (of course, he makes extensive use of ? and !). I tend to think of balloons as more like a blank page of poetry than a blank page of prose — a place that’s fairly wide open.