I think a similar technique was used for the slug as was used for the Rancor- the film was slowed down (or shot at a faster frame rate) so that they would appear more massive and lumbering when the film was projected back.
I think a similar technique was used for the slug as was used for the Rancor- the film was slowed down (or shot at a faster frame rate) so that they would appear more massive and lumbering when the film was projected back.