The big fight sequence was designed, rehearsed and choreographed in the back-yard of director John Carpenter's production office. The fight between Nada (Roddy Piper) and Frank (Keith David) was only supposed to last twenty seconds, but Piper and David decided to fight it out for real, only faking the hits to the face and groin. They rehearsed the fight for three weeks. Carpenter was so impressed he kept the five minutes and twenty seconds scene intact.
The line "I have come here to chew bubble gum and kick ass, and I'm all out of bubble gum" was ad-libbed by Roddy Piper. According to director John Carpenter, Piper had taken the line from a list of ideas he had for his pro wrestling interviews.
Roddy Piper's character never gives his name nor is he referred to by name throughout the entire movie. He is simply referred to as "Nada" in the credits, which means "nothing" in Spanish. The name is most likely a reference to George Nada, the main character of Ray Nelson's short story "Eight O'Clock in the Morning," which was the basis for 'They Live'.
Many movie posters for the film featured a long blurb that read: "You see them on the street. You watch them on TV. You might even vote for one this fall. You think they're people just like you. You're wrong. Dead wrong.".
The film was made and released about twenty-five years after its source short story "Eight O'Clock in the Morning" by Ray Nelson had been first published in 1963. According to John Carpenter, this movie was also based on an Eclipse Comics comic-book adaptation of this story.
According to a title-card in the made-for-DVD short documentary He Lives: Interview with John Carpenter (2013), "They Live opened at the #1 at the US box office. And disappeared from theaters soon afterwards".
The film is partially shot in black-and-white which involved only the scenes and sequences where the aliens were visible to the audience when characters are wearing the sunglasses for most of the film. But this visual aesthetic ceases towards the end of the picture whereupon the aliens become visible in color for the film's final act.