- Ability over Appearance: Jim Broadbent's Slughorn is different from how the books describe him - short, incredibly stout and bald, and with a walrus-like moustache - but he displays the shrewd opportunistic yet friendly quality perfectly.
- Billing Displacement: As has become a common trend of the films by now. The veteran actors are billed in the main credits as usual. Most glaringly, Timothy Spall is listed as a major star despite having only six seconds of screen time.
- California Doubling: The only time in the series this happened; the exteriors for the cave (which is Cornwall in the book) were filmed in the West of Ireland.
- Creator Backlash: Daniel Radcliffe views this as his weakest performance in the series.
- In a note that Alan Rickman wrote to himself during the production of this film, and which was released to the public years later, the actor declares frustration with David Yates, believing that the director "has decided that this is not important in the scheme of things i.e. teen audience appeal."
- Hide Your Pregnancy: Helena Bonham-Carter (Bellatrix Lestrange) was pregnant, but it would have thrown a major wrench into the plot for Bellatrix to appear pregnant. So she spends the film wearing a loose costume and hiding behind the furniture, especially in the Unbreakable Vow scene. It becomes Hilarious in Hindsight when you remember that in Harry Potter and the Cursed Child, she's supposed to have given birth to Voldemort's daughter Delphi.
- Jossed: It was announced that Naomi Watts had accepted the role of Narcissa, but her agency denied the rumors. Helen McCrory played the role.
- Orphaned Reference: The twins Flora and Hestia Carrow are given a lot of attention during the Slug Club scenes. This is a holdover from an earlier draft when Harry would realise that the Vanishing Cabinet had a second one.
- The Other Darrin:
- Frank Dillane replaces Christian Coulson, who played Voldemort's teenage self in the second movie. This was due to Coulson now being viewed as too old for the part, despite him having been so in the first place (23 playing a 16-year-old). Oddly enough, this recasting carried over to the Japanese dub, wherein Jun Fukuyama replaced Akira Ishida as the character's voice.
- Minor example. Lavender Brown had been played by background extras in previous films. Here when she has an actual role, she's played by Jessie Cave - and given a Race Lift (the previous two actresses were both black).
- Real-Life Relative: Tom Riddle's eleven year old self is played by Ralph Fiennes' nephew Hero Fiennes Tiffin. Word of God is that his resemblance to Ralph and his "dark, haunted quality" played a factor in his casting.
- What Could Have Been:
- All the Voldemort flashbacks from the book were in the original script. David Yates however cut them down to Dumbledore meeting him at the orphanage and the two talks with Slughorn.
- Kevin McKidd was offered the role of Fenrir Greyback but he was too busy filming Grey's Anatomy.
- Bob Hoskins was considered for Horace Slughorn.
- Jamie Campbell Bower auditioned to play the teenage Riddle. He didn't get the part but they cast him as the young Gellert Grindelwald for the next film.
- The shooting script had Harry taking Dumbledore's wand after his death. This had to be rewritten once the final book came out.
- Rufus Scrimgeour was originally going to appear but he was saved for the final film.
- This is the first movie since Prisoner of Azkaban to be rated PG instead of PG+13, which is very strange, considering Dumbledore dies in the movie and a surprisingly bloody scene from the novel (Harry cursing Draco with Sectumsempra) is kept intact.
Trivia / Harry Potter and the Half-Blood Prince