Below is a short video on the making of Star Wars: Episode 3 and the amount of green screen that they had to use to obtain some of the effects. At one point in the video they say that there was an effect on every scene in the movie and sometimes dozens. You have to appreciate how much work and effort these professionals put in when you are talking about a feature length film. It takes us hours of fine tuning for very short animations and it is hard to image how much time it takes to complete a film like Star Wars.
I guess I never thought about it before, but I was impressed that basically the whole lava planet was put in through green screen. This means that Lucas had to have the idea in his head of what he wanted for every shot so that the set could be perfect. This just goes to show that you can never overlook preproduction and you can cut down on production times substantially if you have everything mapped out.
