Tools of Characterization
What the characters do in this story (or even what they don't do) ultimately defines them more than what they say or think. The failure to act, on the part of Mrs. McIntyre, Mr. Shortley, and Sulk when they see the tractor rolling toward Mr. Guizac defines them as murderers. Father Flynn's work in helping refugees defines him as a kind man. Mr. Guizac's work defines him as talented and hard working. While we rely heavily on their actions to analyze these characters, many of their actions are ambiguous and will strike different readers in different ways.
The characters in the novel are interested in location. The fact that Mr. Guizac is a foreigner is all some of the characters think they need to know about him. For Mrs. McIntyre (at least at the beginning of the story) "foreign" means "hard worker." For Mrs. Shortley it means "dangerous and scary." While Mr. Guizac is defined by his previous location, the characters on Mrs. McIntyre's farm are defined by their location in the South. While the racism and fear of foreigners certainly weren't limited to the southern United States, the southern US in the 1940s is a specific place and a specific time with unique racial problems and issues.
Speech and Dialogue
O'Connor gives some of her characters filthy mouths. The Shortleys and Mrs. McIntyre are the guilty parties. Their speech defines them as mean and prejudiced. Furthermore, the characters' speech tells us where they are from, and perhaps something about their educational situation as well. Father Flynn's accent defines him as not-from-around-here, as does Mr. Guizac's. The Shortleys' accent would define them for most readers as southern. Lack of speech also defines characters.