Go into the level "Cloud City Trap", and select Free Play. play the level through until the movie where Darth Vader chops Luke's hand off. During the movie, try changing into a character. That character will appear next to darth Vader. You will be able to move around and collect any missed studs in that area. Try doing this with other movies as well. If you can't move try to rapidly keep changing into the same character while pressing the directional buttons to slide around.
NOTE: MAY CAUSE SOME MOVIES TO STOP WORKING. IF THIS HAPPENS YOU HAVE TO RESTART YOUR DS WITHOUT SAVING.