A flat character who does not change throughout the narrative of Steinbeck's novella Of Mice and Men, Carlson is developed through indirection characterization rather than direct characterization which involves direct statements giving the writer's opinion of the character. Here are the ways that Steinbeck does use indirect characterization:
through a physical description of the character
In Chapter Three, Carlson is described as "thick-bodied" with a "heavy-legged stride."
through the character's thoughts, feelings, and speeches
Carlson demonstrates his arrogance and insensitivity in a number of ways. First of all, when he enters the bunkhouse, he assumes everyone thinks it is dark and turns on the second shaded light, commenting in the same breath about the dark in conjunction with the ranch hand with dark skin:
"Darker'n hell in here," he said. Jesus, how that n--- can pitch shoes....He don't give nobody else a chance to win--"
Then, he snifs the air, locating the odor as that of Candy's old dog:
...Get him outa here, Candy! I don't know nothing that stinks as bad as an old dog. You gotta get him out."
Ignoring Candy who "squirmed uncomfortably" and "looked helplessly" at Slim for some "reprieve," Carlson offers to shoot the dog for Candy. When he sees Candy's discomfiture, Carlson callously explains that he will kill him so that the dog will not "even quiver."
"If you want me to, I'll put the old devil out of his misery right now and get it over with. Ain't nothing left for him. Can't eat, can't see, can't even walk without hurtin'."
Then, when Whit enters and shows Slim a magazine with a letter from a former bindle stiff there, "Carlson had refused to be drawn in" to the conversation, indicating his unfriendliness and self-imposed alienation.
Of course, Carlson's insensitivity reaches its magnitude after he arrives at the river bank where Lennie lies dead in Chapter Five as Slim consoles George.
Curley and Carlson looked after them. And Carlson said, "Now what the hell ya suppose is eatin' them two guys?"
through the character's actions
Despite Candy's anxiety, Carlson takes the swamper's old dog and shoots him with his Kruger in Chapter Three. In the final chapter, Carlson eagerly runs for his gun as the men prepare to find Lennie and kill him. But, Slim reaches the river bank before them, and consoles George after learning of Lennie's death. "Curley and Carlson looked after them," puzzled.
through the comments and reactions of other characters
Interestingly, there are no reactions to Carlson as he stays out of any interaction in two instances: in Chapter Three when Whit shows Slim the letter from William Tenner, and in Chapter Five as the men rush in to see Curley's wife lying dead on the hay, Carlson keeps back "out of attention range." Nor, are there any comments made about Carlson's personality. When Curley mentions him, it is only to say that his Kruger has been taken.
From these methods of characterization, the reader comes to realize that Carlson is representative of the alienated men who become hardened, insensitive, even cruell to others.