Steinbeck builds up George's final decision to shoot Lennie in a couple of ways. He starts this build up as early as Chapter 5. When George surveys Curley's wife's dead body and realizes that "—I think I knowed from the very first. I think I know’d we’d never do her," it starts the process of building up the tension to George's actions. It is from this point that he asks Candy to "cover for him" in terms of the other ranch hands. George depicts it as fearing their reaction, but he needs the misdirection to get Carlson's luger. It is at this point that George has made his decision. He knows what he has to do and there is a sad sense of resolve to his actions.
In chapter 6, Lennie illuminates the build up to George's decisions. As Lennie wrestles with the hallucinations of Aunt Clara and the six foot rabbit who both deride him, Lennie moans in the regret of his actions: "Lennie moaned with grief. “I know, Aunt Clara, ma’am. I’ll go right off in the hills an’ I’ll fin’ a cave an’ I’ll live there so I won’t be no more trouble to George." As Lennie has to argue with the rabbit, his own tension and stress increases: "Now Lennie retorted belligerently, “He ain’t neither. George won’t do nothing like that. I’ve knew George since—I forget when—and he ain’t never raised his han’ to me with a stick. He’s nice to me. He ain’t gonna be mean.” Lennie is fighting the idea that George is going to leave him. When George does appear, Lennie exclaims his name in such a way that it serves as almost a confirmation that the worst has been avoided. Steinbeck constructs this moment as George is almost akin to a savior for Lennie. Lennie sees George in an almost heroic light, reflective of the tension and buildup that leads to his decision.
The final way in which Steinbeck builds up George's decision is to have George recite to Lennie their shared dream. When Lennie asks George to “Tell me like you done before," it starts the process in which George's decision is inevitable. There is a build up in the way in which George tells the story, in how he tells Lennie to take off his hat, and in how George directs Lennie to "look acrost the river, Lennie, an' I'll tell you so you can almost see it." Through this build up, George has recognized Candy's words when he says, "I should have done it myself." Steinbeck builds up George's decision as if it is the logical consequence of Candy's warning. He builds up George's decision using dialogue and characterization to make his decision the only one that he could have taken out of love and devotion to his friend.