Believe it or not they filmed the entire thing for real, using all the actors as if it were a movie. Then they did lots of motion capture, and then they 3D scanned all the actors bodies and faces. Then they threw it all into their SX Spectrums and out came the finished product. Cost an absolute fortune, as you can imagine ($165m - but made $301m back worldwide).
The same technology, though even more refined, has been used on the forthcoming 'Beowulf' movie. Can't wait, myself personally, but it's not everyone's cup of tea. You often hear moans of "why didn't they just real actors?" etc. The answer is, they did, of course.
RegarRAB
Mark