You have to remember that Bob Hopkins knew what Roger Rabbit was going to look like and be doing when it was all done. Lucas just told his actors that there will be a monster behind you. Most of the time they didn't even know what it was going to look like. I would be different if they had a picture of what is going to be going on with the blue screen. I can't imagine how hard that would be. Not that I’m defending Lucas I think he could do better as a director.

"There will be a vast army of something behind you. Maybe crab people but that doesn't matter we'll deal with that latter. You just have to look scared and slice at stuff with your laser sword. ACTION!" -George Lucas

And yeah that's not a real quote