The Harry Potter comparison, while not apt in comparing Hogwarts to the Citadel, perhaps rather draws upon the fact that Jim Broadbent is known for his portrayal of Professor Horace Slughorn in Harry Potter And The Half Blood Prince.
Suffice to say that’s a considerable presence in the scope of the seven episode season, and it sounds like Sam Tarly will be along for those parts as well.
Not much is known regarding his character Maester Marwyn beyond him being the archmaester of the Citadel, but Broadbent confirmed he plays a part in around five episodes with a major scene in each.