Actors are always sort of ambivalent about special-effects movies. They know the movie's likely to do well at the box office, but they feel strange about co-starring with the special effects. Alec Guinness observed, for example, that he spent most of the "Star Wars" saga standing in front of a blank blue wall, so that the special-effects guys could put in the visual effects months later. James Caan had a word for the visitors from outer space who were his co-stars in "Alien Nation." He called them "potato heads."