"Oh I beg your pardon Mr Malfoy," Q apologises hastily, belatedly recalling that his host was a Slytherin. "I was once bitten by an unexpected snake," he explains. "The consequences were very...unpleasant." He touches the back of his head.
Q finishes playing the piece. When he's done, he bows his head, very much like he's praying. "Thank you Mr Malfoy," he says simply, not looking up. "The tone is marvelous."