Broadbent will play the potions professor Horace Slughorn, who gives Harry an old tome with a mysterious past. McCrory will play Draco Malfoy's mother Narcissa.
Newcomers Jessie Cave, Hero Fiennes Tiffin and Frank Dillane will portray Lavender Brown, the child Tom Riddle and the adolescent Riddle, respectively. The Riddle character eventually becomes Lord Voldemort.
David Yates, who directed this year's hit Harry Potter And The Order Of The Phoenix, is directing the sixth adventure, which Warner Bros plans to release on November 21 2008. Bruno Delbonnel makes his debut on the series as cinematographer.
'I am really excited that David Yates and all our cast are back for the sixth film and welcome some wonderful new additions to our ensemble,' producer David Heyman said.
'We all share a commitment to Jo Rowling and to Harry Potter fans around the world to keep making these films the best they can be and, as we head into the final installments, I can promise that this level of commitment will not waver.'
Daniel Radcliffe, Rupert Grint and Emma Watson reprise their roles as the three lead characters, alongside Robbie Coltrane, Julie Walters, Ralph Fiennes, Helena Bonham Carter, Michael Gambon, Maggie Smith and Alan Rickman.