Of Mice and Men and Slim

Curley, the boss' son, is an evil character in Steinbeck's world. Even Lennie feels the sense of menace when Curley first comes into the bunkhouse. Curley is a "thin young man with a brown face, with brown eyes and a head of tightly curled hair." According to Candy, Curley is an amateur boxer and is always picking fights, especially with guys who are bigger than he is.
Curley tries to prove his masculinity by picking fights. Another way to prove himself is by marrying a physically attractive woman. His wife is never given a name, but by calling her "Curley's wife," Steinbeck indicates she is his possession. Curley refuses to let her talk to anyone on the ranch, isolating her from everyone and setting the stage for trouble. He makes a big show of keeping his hand soft to caress her, yet patronizes the local whorehouse on Saturday night. While he may strut around the ranch because of his position as the boss' son, he obviously cannot satisfy his wife and is mean to her. Curley beats up any man who dares to talk to her; the only one he listens to and seems to respect is Slim.

Slim is described always in terms of dignity and majesty. When he first comes into the bunkhouse, he moves "with a majesty achieved only by royalty and master craftsmen. He was a jerk-line skinner, the prince of the ranch, capable of driving ten, sixteen, even twenty mules with a single line to the leaders." Slim is tall, ageless, and an expert in his job. His voice is the voice of rationalism. When Carlson suggests killing Candy's dog, Candy appeals to Slim as the final authority.
Slim is so respected and admired on the ranch that even Curley listens to him. When Lennie smashes Curley's hand, Slim is the one who intercedes and tells Curley he will not have George and Lennie fired. Slim understands Curley's fear of ridicule, and he uses that fear to help George and Lennie. Slim also inspires confidences because he is not judgmental. When George first meets Slim, George tells him about Lennie's troubles in Weed. George senses in Slim a person of intelligence and empathy who will not be mean to Lennie, make fun of him, or take advantage of him.
Slim is the only one on the ranch who appreciates the difficulty of George's position. He understands the constant oversight George must exercise in watching Lennie and keeping him out of trouble. It is Slim, in the end, who suggests that George did the right thing in killing Lennie mercifully. He explains the alternative: "An s'pose they lock him up an' strap him down and put him in a cage. That ain't no good, George."
Slim is present at every crucial juncture in the story: the death of Candy's dog, the smashing of Curley's hand, finding the body of Curley's wife, at the pool after George has shot Lennie. In each case, there is violence or the threat of it. Each time Slim helps make the assessment to do what is merciful or what is right.

Carlson is the most arrogant of the men, and the least sensitive to the feelings of others. Though he argues that it would be more humane to put Candy's old dog down, stressing that "he's all stiff with rheumatism...he ain't no good to you, Candy, an' he ain't no good to himself...why'n't' you shoot him...if you was to take him out and shoot him right in the back of the head...right there, why he'd never know what hit him", Carlson's motives are actually selfish. Candy wants the dog gone because its smell offends him; he has no sense of how Candy loves the dog, and how difficult it is to think of putting him down (Chapter 3).
Carlson's lack of sensitivity for the feelings of others are further emphasized at the end of the story, after George, knowing that Lennie is doomed, kills him to spare him from suffering he will never understand. George is understandably distraught after shooting his friend, and Slim empathetically is commiserating with him. Carlson, however, watches the two, and callously wonders, "Now what the hell ya suppose is eatin' them two guys?" (Chapter 6).
One way you might study characterization more efficiently is to color-code when you annotate the text. If you highlight the names of each of the main characters with a different color when they appear in scenes as you read, you can later go back and refer to them individually, review their actions, and develop a clearer picture of what they are like.