How do shifts of scene and sitting serve to express the meaning of the unfolding action in "Barn Burning"?
One setting that directly contributes to the meaning of the action is the courthouse. Not necessarily for its own sake, but for what it means to Sarty. The courthouse is actually important in a scene in which it doesn't even appear.
In the scene during which Abner and Sarty come within sight of the de Spain house, Sarty's reaction is that "Hit's big as a courthouse." He momentarily forgets his father, apparently extremely unusual for him, and even when he remembers Abner, "the terror and despair [associated with his father] did not return." The size of the house compared to houses encountered during the "twelve movings" leads Sarty to think that "They [the owners of this huge house] are safe from him. People whose lives are a part of this peace and dignity are beyond his touch, he no more to them than a buzzing wasp: capable of stinging for a little moment but that's all;..."
This setting (strictly speaking, of course, the setting is the de Spain yard, not the courthouse) and Sarty's reaction to it contribute to the meaning of the work by revealing and making concrete Sarty's associations when it comes to his father; by contrasting the de Spain's home with the shack the Snopes are staying in; by defining Sarty's limited exposure to the world (he compares the home to the only other large buildings that he's seen, apparently (courthouses); and by moving the plot forward to the only complete interaction revealed in the story between Abner and someone whose barn he burns.
Ironically, Sarty is incorrect. Even the de Spains are not beyond Abner's reach. Abner will soon go after their barn, too, which will lead to Sarty's decision to try to stop Abner, and his decision to escape from his family.