Stuart Craig, Harry Potter production designer, recently spoke about working on the series as well as some challenges he had with Half-Blood Prince:
So what, in the Half-Blood Prince, are you excited to see the reaction to -- specific scenes or places?
SC: Well, there's always a new character. And in the case of The Half-Blood Prince, it's Professor Slughorn -- the new potions master. You know, Imelda Staunton was in "Harry Potter 5, and now it's Jim Broadbent in Harry Potter 6. So, one of the biggest challenges is to create the stage and environment for that new character, and Slughorn is a rather theatrical character who enjoys an eventful social life. He kind of lives in the reflective glory of some of his more famous students and invites them constantly to dinner parties. And he's very into lavish parties.... So creating his house, his classroom, his study ... they were the fresh new challenge.
The most spectacular set?
SC: Voldemort has hidden a Horcrux ... The construction of that cave, the design of that cave and entrance is on the ocean in a cliff face beside the ocean. The most spectacular piece of coastline in these islands is in fact the Cliffs of Moher in the west of Ireland.
So having found the entrance, we thought 'What should the interior be?' Well, stalactites and stalagmites. Yeah, they're exciting and sort of interesting formations, but they're familiar. So we found some material about a cave in Mexico that has huge quartz crystals in it. So we began to look into crystal caves ... but [we] also discovered that near Frankfurt in Germany there's a cave with salt crystals in it, so we went off there and it was indeed amazing. It was the cave where Hitler stored all the gold bullion and precious works of art taken from the European countries that he invaded. There were photographs of Eisenhower after the war entering this giant salt mine and discovering all this stuff. But, I'm rambling.