Anthony Goldstein was initially written as a Hufflepuff, but was transferred to Ravenclaw in a later draft.
Rowling knew straight away that she would have to kill off a member of the Weasley family, but didn't know which one. Arthur Weasley was actually originally intended to die of his injuries in this book, but Rowling had a last-second change of heart after realizing that Arthur was the only positive paternal character in the series. She reportedly had an outburst into tears over killing him.
Helen McCrory was originally cast as Bellatrix Lestrange, but had to drop out shortly before shooting began after discovering she was pregnant, and was replaced by Helena Bonham Carter (who, ironically, learned shortly before shooting began on Half-Blood Prince that she was pregnant too).note You'll notice that Bellatrix is wearing looser robes in HBP, as opposed to the form-fitting ones of OOTP. As Carter put it in one interview, she and McCrory "swapped roles", implying that Carter would've played Narcissa Malfoy had the original plan been kept.
Alan Rickman seriously considered stepping away from the series after having been diagnosed with cancer circa 2005-2006, but after the removal of his prostate in 2006, he eventually signed on to return for the remainder of the series.
Anna Friel, David Thewlis' long-time girlfriend, lobbied hard for the part of Nymphadora Tonks, but was turned down.
Saoirse Ronan auditioned for Luna, but was rejected for being too young. Juno Temple also auditioned.
English footballer Theo Walcott and his family were brought to make cameos, as his aunt Yvonne was director Yates' girlfriend. Ironically, Theo himself wasn't able to make it due to commitments with Arsenal FC.
The first script for the film did not include Kreacher. He was added in after Rowling warned the crew this could create serious problems when it came to adapt the as-then unpublished Deathly Hallows.
The St. Mungo's sequence was originally going to be included, complete with an appearance by Kenneth Branagh reprising his role as Gilderoy Lockhart. It was scrapped, because it would have meant building a new set. Instead, Neville tells Harry about his parents personally in the Room of Requirement.
For the brief scene showing Azkaban, the idea came up that putting Azkaban on the edge of a giant waterfall would be a great visual. The problem is that Azkaban is meant to be in the middle of the North Sea. They tried to find some way to justify having a waterfall in the middle of the ocean, such as having a Sea Sinkhole, but eventually they just gave up and depicted the ocean normally.
Sirius' death scene was filmed as it happened in the books, if the original script is to go by (this was confirmed by Helena). The battle scene was, along with so many other scenes, removed (and never released), and the shot of Bellatrix appearing was bizarrely mirrored, as well as dubbing Helena saying Avada Kedavra. Harry's scream was supposed to be heard, but it ended up being muted due to being too agonizing, so much so that it apparently caused Emma Watson, Bonnie Wright, andHelena Bonham Carterto start crying on-set.
The movie was originally three hours long. The final cut is two hours and eighteen minutes, and only about ten minutes worth of deleted scenes were included with the DVD extras. This leaves at least a half hour of unreleased deleted footage.
stepping away from the series after having been diagnosed with cancer circa 2005-2006, but after the removal of his prostate in 2006, he eventually signed on to return for the remainder of the series.