"That same smile seemed to threaten, the one that was just a little bit wise, like he was remembering the punchline to a particularly filthy joke."

I loved this line since it describes Sirius so very well.

I also loved how, in the 2nd to last paragraph, you nail Hermione and her active thought processes, and how Sirius' kiss short-circuits it. Well done.

More than anything I really liked the mood, pacing, and voice. Beautiful.