A number of years ago I think it was Entertainment Weekly did an article about "Aliens." Paxton's "Game over, man!" line was ad-libbed by the actor. But he explained that it was not an ad-lib on the set. He said that unlike most movies, "Aliens" was cast sufficiently in advance that they actually had rehearsals. He said that the cast read through their lines a number of times before filming, and director James Cameron was open to suggestions. So the actors had input, and Paxton suggested that line.