JC: We found that lighting the blue skin was very difficult, especially when you start to add sweat and oil sheens to the surface of the skin, which all human skin has. There was a point at which the blue skin could very quickly turn plasticky if you weren't careful. And the normal lighting cues that you'd have with human skin we studied. We actually went to a rainforest in Hawaii and studied how the light reflected off the plants and how the face takes light from the sky (whiter parts of the sky; bluer parts of the sky) and the interaction of colors off the face, except we did it all with human faces. And then we tried to apply those lessons to blue faces and they didn't work because the color relationship between the key and fill sides of the face in daylight is usually a shift from white light to blue -- and you just couldn't see it. So we wound up experimenting with other ideas. And if you study the film, you'll see that the bounce light is actually a green light, so we took this conceit that sunlight was bouncing around in the forest and it was reflecting back green, or transmitting green in the leaves or whatever. And there was this green ambient light at all times. And then there was a mixture of blue skylight and white sunlight in play with that. So there are actually three colors of light used to light the Na'vi faces but only two of those colors -- the green and the white -- are really creating enough of a color difference to work. And that was something that Joe Letteri [the senior visual effects supervisor from Weta] and I worked out very early on; actually, in the very first fully rendered scene that they did. But it wasn't intuitively obvious that that's how it would work.

The blue Na'vi proved problematic, so they used green bounce light in conjunction with white to properly convey the faces.

BD: How did you achieve that "critical moment of truth"?

JC: I had this philosophy of life: I actually borrowed it from Arnold Schwarzenegger because he used to say you program yourself for success, not for failure. And at first I didn't understand what he meant. But the more I thought about it, what I realized what he was saying was that every decision you make as if you were going to be successful because, in his world view, you're never going to fail. Take that idea and apply it to performance capture. So I go in and say, all right I'm going to go down this road and spend all of this money and we're going to do this. The success would be defined as: 100% of what the actor does arrives, finally, in this CG character's face at the end of the day. Let's take that as a given. Then here are the ground rules: you don't over act; you do the most subtle thing that you feel is correct for the moment. You don't try to modify the acting process for this imagined result down the line.

That's one rule. The other rule is: as a director, I don't leave the set and walk away from the actors until we have the exact nuanced performance we think we need. We're not going to try and fix it later; we're not going to try and embellish it with animation. We're going to assume that we are neither going to lose any information nor add anything. And if you go by those assumptions, which were my assumptions going into this, even though it's fairly cheeky because this had never been done, it exactly defined how we worked. Which meant that it was highly focused on getting the right performance, which is why Sam [Worthington] and Zoe [Saldana] are so damn good in the movie.

BD: Technology caught up with need.

JC: It was more like flogged forward with bull whips.

BD: Let's discuss the virtual environment breakthrough.