The famous "Chew Bubblegum" line was, in fact, an adlib by Roddy Piper. There's also the very, very long fist fight between Nada and Frank. It was supposed to be cut down, but the creators enjoyed the fight so much, they decided to leave the whole thing in.
The entire fist fight scene was always intended to be ad-libbed. Carpenter's shooting script simply said "FIGHT CONTINUES" for six pages, having the two actors do their thing.
During said fight, they also really hit each other. The only thing faked were the face and nut shots.