BD: And it was important to visualize air, water, earth and fire. I understand you were concerned about CG fire?

MNS: Yeah, my original concern was that fire does not look real on film, even if it's real fire. It looks improbable and doesn't follow the rules of lighting and physics that you intuitively know. So I didn't know how we were going to circumvent this. But ILM eventually found a language and a movement and an understanding about that small band where fire looks good and believable. That turned out not to be an issue.

BD: What about changing the fire bending rule from the series, making it based on a physical presence and not coming out of nowhere?

MNS: Yeah, when I watched the shows that was just stuck in my craw as something that wasn't explained and that the rules weren't even for the four nations. It was an interesting idea to turn your chi into energy. But I felt that should be reserved for the highest fire benders. Each movie ups what people can do with the bending of the elements. So that's what I also love about the progression of the movies, until you get to the third one where you have perversions of how non-ethical people start using bending.

BD: And air proved too abstract?

MNS: Yes, that was an R&D process where they eventually found samples of smoke that were moving in wind tunnels and things that created a movement that I thought was ghostly and interesting and wispy: churned air that creates this kind of foam. That was the principle. And beyond that, it had to be from the environment. Air ended up being solved pretty quickly as well.

BD: Earth was a matter of particle simulation but what about water?

MNS: Water was the most difficult because it had a lot of physics involved. I wanted to get that kind of astronaut thing where they take the water out of the jug and let it float in front of them. I wanted that kind of effect and it took a long time to do each and every one of them.

Appa stayed true to the series, which was Shyamalan's biggest leap of faith.

MNS: I grounded everything in a reality that I could defend and believe in; and Appa [flying] I couldn't get my head around, so I said we're just going to have to take a leap of faith. Otherwise, I think we would change the spirit of the show too much if we changed that character.

BD: And Momo, which we don't see a lot of?

MNS: Yeah, Momo's pretty cool. The second movie is pretty creature heavy and, hopefully, he will work his moments in a more significant way in the future. But I actually think he would exist in the real world.

BD: And the spirit dragon?

MNS: It's more of a Chinese dragon than a European dragon.

One of the sculptors made a beautiful maquette, and it was this long, snake-like thing, and I said, "Let's put that in a cave and coiled up, and only at the end, uncoil itself and walk by the boy and see it in its glory."

BD: What about the impact of the last minute 3-D post conversion?

MNS: Yeah, the last-minute decision for 3-D was basically a year in the making. It's an announcement that happens late in the game by the nature of the process. They can't do tests until you have stuff to show them from ILM, and I can't make a decision until you show me many tests. They can't even work on it until the movie is completely picture-locked. So it's always going to be late in the game from a conversion standpoint.