The actors playing the Colonial Marines in Aliens were required to read the Robert A. Heinlein novel Starship Troopers as part of their preparations prior to filming, and also took part in a training boot camp run by the SAS (Special Air Service), the UK's elite special forces regiment, to help them effectively portray military combat personnel on screen. During this time the actors each personalized the armor worn by their characters. The notable exception being Michael Biehn who inherited his armor from the actor he replaced (and also subsequently missing the boot camp).