Not In the Script
The parody was iconic for all its witty lines, but there was one that stood out. After the townspeople showed their hate towards Bart, Waco Kid consoled him by saying: “You’ve got to remember that these are just simple farmers. These are people of the land. The common clay of the new West. You know… morons.”
Source: YouTube
That last part of the line, “you know… morons,” was ad-libbed by Wilder and the improvisation caused Little to genuinely crack up.