Last night, Nance and I were watching a bit of Hitchcock's To Catch a Thief and we got into a discussion of rear projection. For those that are not familiar, this was a technique that was used before blue (and subsequently green) screen chroma keying techniques for replacing elements in a shot or providing backgrounds. Basically, a large translucent screen was placed behind the talent and a projector whose shutter was synchronized to the camera projected footage onto the screen. This was used a great deal for car interior shots where everything was shot on a soundstage, the car set was mounted on springs, and grips would rock the car to simulate the car driving down the road.
Nance pointed out how obvious this technique was in the films. We made a game out of guessing what scenes were using this technique and to be fair, there were some scenes where it was pulled off very well. Others, not so much. Today, audiences are MUCH more sophisticated and demand a much higher quality of compositing in films. To give you an idea of just how much this technique is employed in popular content, check out the demo reel for Stargate Studios here:
Also note that not ALL of this is green screen. There is also a lot of matte shot replacement techniques in use here as well.