(Warning: Potential spoilers for Season Seven and Game of Thrones follow.)
What does this revelation mean? It's hardly surprising that Broadbent was recruited to play an "old professor character" since he's perhaps most famous around the world for playing potions professor Horace E. F. Slughorn in the Harry Potter films. (I didn't think he'd be playing a sword-swinging knight or a dragon.) But, as ScreenCrush's Erin Whitney points out, it's likely that Broadbent will be playing a specific important professor: Maester Marwyn, who readers of author George R.R. Martin's novels will be familiar with.
Of course, it's possible he could be playing a maester with a different name: Game of Thrones writers David Benioff and D.B. Weiss have been known to deviate from Martin's writings on occasion, particularly in the most recent seasons. However, if they stay true to the path presented in A Feast for Crows, Marwyn could end up being a pivotal character in the show's endgame.