Movie Magic: Is there anything about Harry Potter and the Half-Blood Prince that stands out for you in terms of challenges that had to be met?
David: One of the big challenges, funny enough, is that we had a lot of night exterior shooting in the English winter, which is a huge challenge. But technically speaking, we have things like the underground cavern at the end of the film where Harry and Dumbledore go on one of their journeys. Technically, it's very difficult and challenging to pull off, because it's another digital set. In visual effects terms, the cavern is the most challenging aspect of the film, which is fun. It's a tremendous sequence because in it we have the inferi, who are the zombies of our world.

Movie Magic: Is that worrisome at all given the fact that there have been so many movies in the last few years that have focused on zombies?
David: To try and come up with something that is not a generic zombie and not something that immediately makes you think of George A. Romero [who directed Night, Dawn and Day of the Dead] is difficult, because the moment the words 'living dead' are mentioned, the image that springs to mind is things like Romero and Simon Pegg [who helmed the Shaun of the Dead movie]. Obviously, that's the last thing one wants to conjure up. So that's been really challenging, but I think we've come up with something quite special and scary. I think it's going to be an exciting sequence.

Movie Magic: Have you been signed to the two-part adaption of Harry Potter and the Deathly Hallows?
Jany:Yes, and it's a challenge too, because its the end. You have to do the finale in the best way you can. It's very challenging for me. I'm quite afraid, actually, because I always felt that doing a number five or a number 6, you could always do better next time. Now this is the last one, so I've got to do my best. This is the end. It's always difficult to style a finale, because it is the last one that people will aways remember.

Movie Magic: For this film, Half-Blood Prince, what has been the most challenging part for you?
Stuart: J.K. Rowling always writes a big scene in a big venue, and this time, Harry and Dumbledore go off into a mysterious cave, and that cave is a big challenge. We only physically built two tiny little parts of it and the rest is a computer-generated set, but for that, we made a small scale model, a huge-scale model and, as I sad, a small physical part of it. Films are made differently these days. Those models are then scanned, and the scans of those physical and three-dimensional models become the blueprint or the structure on which the CG set is finally made and rendered. It's a challenge that it needs to deliver dramatically and yet have this fantastic otherworldiness about it. It's almost entirely computer-generated. Hardware is comparatively easy and organic rocks forms less so. And it's a challenge for somebody of my generation, frankly, to catch up with the 21st century movie technology.

Movie Magic: Is there a secret to being able to do what you do and, as you said, catch up to modern movie technology?
Stuart: A key to what we did this time was our initial research. We started by saying, "What's interesting about a cave?" Well, stalagmites and stalactites are probably the most familiar, almost cliched, of limestone caves. So we started to examine crystal caves and looked to the pictures of this incredible quartz cave in Mexico. We did visit one in Switzerland and thought, "What other kind of crystal caves are there?" It turned out to be a salt crystal cave outside Frankfurt in Germany, so we went there, and it's part of a cast phosphate mining complex, but there's also rock salt down there, and in this one area salt crystals, which are completely transparent and glass-like. We took extensive photographs and used that as our inspiration. What it's done is given us this hopefully fantastic set and fantastic world but with a bit of geological credibility at the same time, which we've always tried to do. Yes, it's a magical world, but it's grounded in a recognizable reality. We've treated the cave as an extension of the architecture in that respect.

Movie Magic: Because you have actors performing against the small parts of the cave you have to actually build, what aspects of it did you construct?
Stuart: We built that island, we built the boat that they travel in, and part of the shoreline they leave to get to the island. But they are like one percent of the total that you'll see in the final film.

"Each has to be a self-contained unit. Each film so far has been different from the one that preceeded it, so we need to do that with Deathly Hallows Part 1 and Part 2. There is the challenge of making two separate films that you want to have an identity of their own while you're shooting them at the same time. That being said, what is exciting, what works in our favor, is that we have a very clear sense of what the two films are and what the thematic underpinnings of each part are, which in itself will provide a distinct nature to each of them."

"It's crucial that they feel distinct, even though you're never going to be able to separate them entirely in the sense that Part One is the beginning of the end and Part Two is the end of the end, even though stylistically they should feel like two very different movies. If you look at the landscapes they're likely to occupy - I say 'likely,' because we don't have scripts yet thanks to the writer's strike - they will be different. Think of the first part of the book. They don't arrive at Hogwarts until the final denouement of the first part. In some respects, the first part of that book feels like a road movie. They're harried and harassed and they're trying to travel and hide themselves in the Muggle world with only the flimsiest of charms to keep them out of the site of the enemy. That in itself is a very different feeling from anything we've had before."