In an interview with Dark Horizons, actor Jim Broadbent revealed who he will play in Harry Potter and the Half-Blood Prince, which started filming on Monday for a November 21, 2008 release. Here’s a clip from the interview:
Who am I? He’s called Horace Slughorn. He’s a retired teacher of magic who’s drawn back out of retirement because he’s got some secrets they need in the battle against the Death Eaters and he’s quite star struck as a teacher and he’s drawn back into the fold because he likes to notch up celebrity students, and he’s drawn back by Harry.
He talks a bit more about the character, as well as his role in Indiana Jones and the Kingdom of the Crystal Skull, at the link above.