Also, don't forget that Cain's re-entrance in S10 which I talked about in my One Scene Each Season series also counts as an entry in this series!
Charlie (and Sam&Dean) - LARP and the Real Girl (8x11)
So, this might be stretching the idea of an "entrance" but I think it still counts. It's not the first time we meet Charlie, and it's not even the first time that we see her in the episode (we see a recording of her at the police station) - but it IS when she re-enters the narrative as an active participant... so, to me, that's an entrance. And it's definitely filmed as one. If this were a play, this would be the first time that Charlie was on stage, so, entrance.
And it's a great entrance - it's short and to the point. First, we get the reveal that she's not just supervising the knight tryouts, but she's testing the knights herself. We get the "I am no man" helmet removal reveal, ala Eowyn, and then the Han Solo reference that's a signature of Charlie's character as well... both things tell us, or remind us, firmly, who Charlie is. She's a nerd, sure - but she's also skilled and charismatic.
That charisma is immediately thrown into contrast with her faltering when she sees Sam and Dean - and this is why this is one of my absolute favourite entrances (possibly my actual top one favourite) because this isn't just an re-entrance for Charlie, this is a re-entrance of Sam and Dean into Charlie's life, and we actually GET TO SEE IT FROM CHARLIE'S POV. Sam and Dean are just standing in the crowd, sure, and Dean gives that little chagrined smile, but they're also clearly visible as having KNOWN MEANING. We don't just see Sam and Dean in out-of-place FBI suits, we see Sam and Dean as people like Charlie see them - foreboding portents. If Charlie's missing players were symptoms, then Sam and Dean arrive in the crowd like a diagnoses.
I love this entrance because it does double-duty. It's a great example of a short-but-effective character re-introduction, but it's also possible the only time we get to see a Sam and Dean entrance from the point of view of a civilian who knows about both the Supernatural and the Winchesters.