In an example of dramatic irony, readers are aware that Max and Al are holding everyone in the diner hostage while they wait for Ole Andreson to arrive, but the few patrons who come into the diner in the intervening hours are completely unaware of this fact. The irony comes across in the following passage, as the narrator describes one such customer arriving for a to-go order:
Once George had gone out to the kitchen and made a ham-and-egg sandwich “to go” that a man wanted to take with him. Inside the kitchen he saw Al, his derby hat tipped back, sitting on a stool beside the wicket with the muzzle of a sawed-off shotgun resting on the ledge. Nick and the cook were back to back in the corner, a towel tied in each of their mouths. George had cooked the sandwich, wrapped it up in oiled paper, put it in a bag, brought it in, and the man had paid for it and gone out.
This passage captures the cool and collected way that George goes to the back of the restaurant to make a simple ham-and-egg sandwich for a customer, doing so while winding around Al holding “the muzzle of a sawed-off shotgun” and Nick and Sam sitting in the corner with “a towel tied in each of their mouths.” The mundane way in which the narrator describes how George “cooked the sandwich, wrapped it up in oiled paper, put it in a bag, brought it in" captures the irony of him acting normal in completely abnormal (and quite threatening) circumstances.
While this customer who ordered the ham-and-egg sandwich expects the diner to be functioning normally—and leaves the premises still believing this to be true—the reality is that a violent crime is taking place in the background. This is one of the many moments in which Hemingway highlights the tension between expectations and reality.