Dame Maggie Smith completed filming this movie while undergoing radio-therapy as treatment for breast cancer.
Hero Fiennes Tiffin was cast as Tom Riddle, age eleven, while his uncle, Ralph Fiennes played Lord Voldemort (formerly Tom Riddle). His parents are Martha Fiennes (Ralph's sister) and George Tiffin. Director David Yates says that he hired Hero because of his resemblance to his uncle, but not specifically because he was the actor's nephew. He liked the dark haunted quality about the young actor.
Daniel Radcliffe's least favorite own performance in the franchise, stating in an interview in 2014 that he was "just not very good in it", and calling it "hard to watch". He also revealed that he was struggling with alcohol at the time of filming and can tell in certain scenes that he was intoxicated.
Robert Knox (Marcus Belby) was tragically stabbed to death on May 24, 2008, just a few days after filming wrapped.
Emma Watson considered not returning for this movie. She often felt a bit lonely on set, as being a girl in a group of boys made her feel as much an outsider as her character, and she had difficulties living with the fame that came with the movies. However, she eventually decided that "the pluses outweighed the minuses", and could not bear to see anyone else play Hermione.