well i think the difference with Roger Rabbit and STar Wars is that in Roger Rabbit although they werent acting with anyone at some points... they still had a real atmosphere...

they had a set around them... the could bump into chairs or car or buildings... something to base their performance around..
in Star Wars... most of the time they had nothing.... and when i say nothing... i mean absolutly nothing!!!
just green screen....
i dont see how it is possible to base a performance around that...
at least a good one anyway