It's a loaded situation that could have gone into a hundred half-formed plot threads, but instead Dan and Drey merely become confidants. Drey's father is absent, and her mother is forever pulling double shifts, so Dan is the closest thing she has to a role model. Not to mention that Drey is all too familiar with drugs (her older brother is in jail, presumably for that reason); they don't particularly spook or offend her.
Fleck and co-writer Anna Boden stage Dan and Drey's conversations with a jazzy rhythm. Dan keeps a teacher-like distance, trying to impart as much wisdom as he can in answer to her questions; he's fallen in stature, but also gained, given that they've become closer friends.
The film gives Drey and Dan their own breathing time apart from one another. Drey begins to attract the attention of a charming neighborhood drug dealer, Frank (Anthony Mackie). It's this relationship that sets off alarm bells for Dan, and the amazing confrontation between the two men winds up turning back upon Dan.
Essentially a buddy movie—but one in which the buddies sacrifice genuine parts of themselves—Half Nelson is the kind of movie that Good Will Hunting could have been. It prizes intelligence over likeability, ambiguity over manipulation. Such a film ought not to be such a rare surprise.