14 Harry Potter actors who have appeared in Game of Thrones

Potter: Hermione's mum, a dentist with a wiped memory.

Thrones: Catelyn Stark, badass mother murdered at the Red Wedding.

Potter: Aragog, giant spider (actually an acromantula).

Thrones: Grand Maester Pycelle, slippery old scientist, historian and politician, hacked to death by feral children .

Potter: Amycus Carrow, Death Eater sent by the Ministry (and therefore Voldemort) to infiltrate Hogwarts' teaching staff.

Thrones: Dagmer Cleftjaw, top reaver of the Iron Islands.

Potter: Aberforth Dumbledore, Albus' grumpy brother.

Thrones: Mance Rayder, leader of the free folk. Burned alive by Stannis Baratheon and put out of his misery by Jon Snow.

Potter: Professor Horace Slughorn, socially ambitious Hogwarts teacher.

Thrones: Archmaester (name so far unknown), Sam's learned master at the Citadel.

Potter: Cormac McLaggen, vain, cocky Quidditch hero.

Thrones: Dickon Tarly, Sam's brother (and favourite of their father). 

Potter: Oh yeah, that guy – Ministry of Magic employee with a fire-breathing chicken.

Thrones: Oh yeah, that guy, one of the wealthy elders of Qarth. Killed by the warlock Pyat Pree.

Potter: Bit of a cheat this one – the extremely tall Whyte played part-giant Madame Maxime in shots where you can't see her face.

Thrones: Wun Weg Wun Dar Wun, best giant there ever was. Died in a hail of arrows after breaching the gate of Winterfell for Jon Snow.

Potter: One of Draco's nasty sidekicks.

Thrones: The deserter from the Night's Watch who witnessed the first White Walker murders in the first ever scene  and was executed by Ned Stark.

Potter: Mr Borgin, owner of Borgin and Burke evil magic shop on Knockturn Alley – only appears in deleted scenes.

Thrones: A ranting street preacher.

Also memorable for hosting The Crystal Maze and briefly being singer of The Sex Pistols. Quite the career. 

Potter: Irma Pince, the Hogwarts librarian

Thrones: A Braavosi woman watching the bawdy players' performance.

Potter: A Ministry wizard.

Thrones: Lothar Frey – one half of the two idiot Freys who murdered Catelyn and Talisa at the Red Wedding. Ended up fed to his father in a pie.