You have something real to work from like Gandhi with historical references, but how do you come up with historical references for scenes in fictional worlds -- like the memory scenes in "Harry Potter?"
SC: It's a fairly haphazard process ... The starting point is always the books, you know. She [J.K. Rowling] describes where he [Dumbledore] keeps the memories in these little glass vials -- so that's a given. And then, since the style of Hogwarts has been established as this very antiquated medieval style, and Dumbledore's office is like that, I began to look through books of church architecture, cathedral architecture ... and I delved in there for this Gothic look. The reference I found was for a font. So I adapted it.

The memory itself is poured in to a basin and we thought that a highly reflective liquid surface, like mercury, would be most effective.

What substance did you actually use for the memories?
SC: It was a computer-generated, CG effect, as many of these things are. These movies have been done for nine years now. When we began, there was much more done physically -- and it's amazing when you look back on the progress of digital effects in the last nine years.

When dealing with CGI, do you suddenly see some things and think 'that would be better computer-generated?' Or do you just design everything as if you will physically have to build it?
SC: A lot of that has to do with physical limitations. Obviously the actors would prefer a physically real set to react to and respond to as directed. The CG effects encourage you to be bigger and more ambitious and scale things up, so what we try to do is build a physical set -- which is always limited by the soundstage it's on or the corner of the soundstage you have available -- and then the CG extension becomes hugely important. Most of our sets when we began "Harry Potter" were real or were fully realized prototypical sets, and now most of them are part built with a bigger part that's a CG extension and then occasionally and increasingly, some sets are almost totally computer-generated. We just build the floor that actors stand on.

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.

No, no ... that's very cool.
SC: The Horcrux is hidden in a hollow in the middle of a lake in this cave, and it's vast. Then came the question about how much do you build physically and how much would be computer-generated extensions. We did go quite large, in two quite large chunks physically -- but again, there is a physical limitation governed by the stage that you're in. But it is a massive digital construction.