The trick is she's got SO much fodder for storylines at this point. A big cast of characters, all she has to do is pluck one out and write him in as a more major character in the next tale.
Who knew Neville would be such a big character when we first hear his name from Hermione on the Hogwarts Express, when he'd lost his toad??
I'm working sort of simultaneously on books two and three of my series, and I've found out how really neat that is to do. One character in book one, who has very minor amoung of dialouge, she's gone after half a page, will be big in book three.
It's SO cool to do it. When I thought of it, it was light a light clicked on, and I had a flash into JK Rowling's mind. Not really, but it must be what she feels like.