This is the sixth outing for Harry and throughout the films the central characters have, for the most part, remained the same. Each new film does always bring new characters to the table and further opportunities for new acting talent to get onboard. There are also changes of personnel behind the camera and while David Yates, who directed the last outing, ended up directing this as well it was not a foregone conclusion.
There were several directors mentioned in connection with Harry Potter and the Half Blood Prince. Terry Gilliam was approached to direct and he was actually author J. K. Rowling’s first choice to direct the original film. Gilliam declined and apparently said “Warner Bros. had their chance first time around, and they blew it.”
The next director to turn it down was the talented Guillermo del Toro who decided he’d prefer to work on the Hellboy sequel. Both Anand Tucker and Michael Hoffman were touted as potential directors but the decision was finally made to stick with David Yates.
The most contested part, perhaps surprisingly, was the role of Lavender Brown the love interest of Ron from the book. Over 7,000 girls auditioned for the part but in the end it was all a waste of time because Emma Watson, who plays Hermione, recommended Jessie Cave. She won the role without even attending any of the auditions.
Ralph Fiennes real life nephew, the bizarrely named Hero Fiennes-Tiffin was cast as Tom Riddle. Director David Yates said he chose him because of his resemblance to his uncle, Ralph who plays the adult Lord Voldemort. Christian Coulson was apparently keen to return to the role he played in Chamber of Secrets but Yates felt he was too old for the part. Jamie Campbell Bower also auditioned for the role but ended up winning a part in the forthcoming Harry Potter and the Deathly Hallows instead. The older Riddle was played by Frank Dillane despite interest from Thomas James Longley.
There were widespread reports that Naomi Watts would be playing the role of Narcissa Malfoy but her agent denied it. The part went to Helen McCrory who had actually been cast previously as Bellatrix Lestrange in Harry Potter and the Order of the Phoenix and then had to pull out due to pregnancy.
Lots of actors reprised their roles from earlier films including Dame Maggie Smith who filmed despite undergoing treatment for breast cancer at the same time. Bill Nighy was keen to play Rufus Scrimgeour but he didn’t make an appearance in the film. Eleanor Columbus played Susan Bones again but her scene was cut from the final edit. Timothy Spall plays Wormtail again but he doesn’t get a single line of dialogue here.