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.
The fight between Nada (Roddy Piper) and Frank (Keith David) was only supposed to last 20 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 5 minute and 20 second scene intact.
John Carpenter wanted a truly rugged individual to play Nada. He cast wrestler Roddy Piper in the lead role after seeing him in WrestleMania III. Carpenter remembered Keith David's performance in The Thing and wrote the role of Frank specifically for the actor.
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'.
The Cripple fight in episode 67 of South Park that aired on June 27, 2001. was taken blow by blow from the fight between Frank and Nada in the alley. If you watch it its a match scene for scene of the alley fight
Roddy Piper, being a married man at the time of filming, refused to take his wedding band off. That's why in several scenes you can see a wedding ring on.
The trivia items below may give away important plot points.
When Frank (Keith David) angrily asks Nada (Roddy Piper) how many people he had killed, Nada replies that they weren't people. The only human character killed by Nada is Holly Thompson (Meg Foster).