"If you look at a fire, it's actually pretty difficult to tell how deep it is. By piling up a bunch of slices together, you still get dimensionality to it but you're giving your time and energy to the screen space, where you need it, like the edges of the flame where you want to see those little licks."

Yates emphasized that the Inferi not look like grotesque zombies — rather, they appear very skinny and sickly and make Gollum seem like a he was on steroids. Yates stressed that this sequence should not feel like a horror movie — he wanted audiences to feel sorry for these emaciated, water-logged people. "We really investigated how to not make them look like zombies," Alexander says.

For these characters, ILM needed to avoid Gollum and his ilk, including recent zombies such as those found in I Am Legend. "You pull reference from all those movies so everybody's conscious of what's out there and what you don't want to do," Alexander says, "and try not to make a dupe of those."

Yates cast a few live actors to act out some of the Inferi movements — like overwhelming Harry and dragging him underwater — and ILM animators then made the undead individuals move in a very methodical, creepy way. The actors fight with Harry and pull on him but ILM subsequently painted them out and replaced them with CG Inferi.

Marc Chu, ILM's animation supervisor/director, wanted to take on as much animation as possible — the shots with up to 100 Inferi in them are actually all keyframed and handled by the animators. "Once we get underwater and start to get into thousands of them, that's when we went for a particle-spray approach," Alexander says.

In fact, everything about the cavern sequence is CG — even the eerie liquid that Dumbledore must drink from the Horcrux.

ILM had to build an entire CG environment where the camera could go anywhere and still look like it was in a crystal cave. Just the crystalline background scenery took seven to eight weeks. The crystal surfaces reflect Dumbledore's fire as well as the characters in the action and the transparency allows you to glimpse action through the crystals.

"The lake starts to boil," Weaver says. "We had to create the look of boiling water and the Inferi would be refracted through the bubbles. The fire also penetrates through the water's surface, generating a lot of bubbles."