Lev Grossman: How Harry Potter Became the Boy Who Lived Forever:

Fictional worlds, while they appear solid, are riddled with blank spots and unexposed surfaces. There’s a moment toward the end of Harry Potter and the Goblet of Fire when Dumbledore suggests offhandedly that Sirius Black should “lie low at Lupin’s” for a while, referring to Harry’s former teacher Remus Lupin. What exactly did Sirius and Remus get up to there, chez Lupin, while they were lying low? How low did they lie? (Cough, slash, cough.) Rowling never says, but that one little gap has given rise to so much fan fiction that “lie low at Lupin’s” has become a recognized trope of Harry Potter fan fiction, a sub-subgenre in its own right.

Fantastic essay, and wonderful research from Lev Grossman of TIME.