He teaches Potions, but he doesn't want to—everyone knows he's after Quirrell's job. Knows an awful lot about the Dark Arts, Snape.
From time to time I stumble upon this type of speech with comma + name after the main text to emphasize on a person. So, pronoun-text-comma-name.
Is this a specific dialect? Like, for example "I gonna kill'em" sounds southern-ish US.
English is not my mother tongue though I read a lot and such subtle things are of highest interest to me.