In the scene where Roy L. Fuchs (Jack Warden) walks up to Rudy Russo (Kurt Russell) and Jeff (Gerrit Graham) as they are finishing shoveling dirt over the spot where they buried Luke, Graham didn't have any lines and kept pestering Robert Zemeckis and Bob Gale for lines. Finally, he just decides to repeat whatever Russell said to Warden. Apparently, Warden was unaware of what Graham was doing, thus his line "What're you? A fuckin' parrot?" It was Warden's genuine annoyance at Graham, which worked so well in the scene that it was included in the final cut.
According to Bob Gale in the Blu-ray commentary, executive producer Steven Spielberg was appalled by Rudy Russo's line about President Jimmy Carter: "Hey, he fucks with us..." Spielberg was a supporter of Carter. Robert Zemeckis and Gale refused to remove the line, because where they came from, (Gale is from Missouri and Zemeckis is from Chicago) politicians are (in their opinion) corrupt.
The reason Used Cars (1980) was made at Columbia Pictures was that Universal Pictures passed on this, and since the head of Columbia had once sold cars, he understood this right away.
In the scene in which Gerrit Graham's character starts shooting at Roy L. Fuchs' cars for the commercial, real ammunition was used to shoot the windshields and the headlights.
After the commercial where Jeff shoots up the cars on Roy L. Fuchs' lot, he is interviewed by the FBI. In the scene, he blames the attack on Iranian students. At the time of the movie's release, the Iran hostage situation was going on. According to Bob Gale in the Blu-ray commentary, an alternate version of the scene was shot in which Jeff blames a different group for the act in case the Iran situation was resolved before the movie was released. It wasn't, and Jeff's false accusation was still relevant when the movie came out.