What evidence suggests that the robbers are indeed following a script of sorts throughout the robbery?

Anders's assessment of the bank robbery proceeding according to a "script" (8) is given some credence when a particular action is taken by the man with the pistol. At one point in the story, just before Anders's use of the word "script" (8) in fact, the man with the pistol yells, "'Keep your big mouth shut!'...though no one had spoken a word" (6).