"But they are more metaphysical than physical; they attack the mind, yet if you touch them, they disappear. So we started with black cloth swirling underwater, and created a computer-generated image from there."
The article also includes a description of a visit to the set, with quotes from several members of the cast.
Radcliffe skips happily up the hill, ready to run again. He grins at Hermione, Emma Watson, who is in the background talking in sign language to the crew. She yawns. Radcliffe spins his wand like a cheerleader. Across the fake lake, Gary Oldman, who plays Sirius Black, appears to be having a little picnic from a brown bag. Soon he lies down on the hill, apparently asleep.