Twenty-five years have now passed since the first Harry Potter film and, with the HBO reboot due out this Christmas, Warner Bros is ramping up the celebrations. Key among them is the unveiling of a new feature at the studio tour showcasing key moments, costumes and props from Harry Potter and the Philosopher’s Stone. And this is why Mark Williams is now taking your questions – although, as Potter purists will know, his character doesn’t actually appear in the first film. Arthur Weasley does, however, play a pretty big role in the other seven movies, so let’s muggle through regardless. Julie Walters with Williams in Harry Potter and the Chamber of Secrets. Photograph: Collection Christophel/Alamy In the movies, Williams plays the ministry of magic employee, husband to Julie Walters’ Molly Weasley and father of Ron, Ginny, Fred, George, Percy, Charlie and Bill – a role for which he had to dye his hair red.…