What should be understood about the following characters in Of Mice and Men: Slim, Whit, Carlson, the Boss, and Curley?

Slim is an important man on the ranch. He is a "skinner," meaning he drives the teams of horses and mules during harvest time. More than that, he is the de facto leader of the men. He is definitely more important to the working men than either the boss or the boss's son Curley. Steinbeck refers to him as royalty and almost godlike. There is an excellent description of him in chapter two. He plays two important roles in the novel. He makes important decisions and he is a sounding board for George. He also offers important advice to George after Lennie kills Curley's wife.

Slim acts as the final word in the killing of Candy's dog. Candy is hesitant about putting down his longtime companion despite the fact the dog has grown quite old and decrepit. He is suspicious of Carlson's motives, but Slim makes the decision final. Steinbeck writes, "Candy looked helplessly at him, for Slim’s opinions were law." Slim is also a good listener as George tells about his life with Lennie. George confesses that he used to treat Lennie badly but they are really close friends. Slim is portrayed as all knowing and he understands the importance of the friendship between George and Lennie.

Slim is responsible for George and Lennie keeping their jobs as he dissuades Curley from saying anything about the fight in the bunkhouse when Lennie crushes Curley's hand. As with the dog, Slim also provides George with advice about Lennie. After Curley's wife is found dead in the barn, Slim says,

"But Curley’s gonna want to shoot ‘im. Curley’s still mad about his hand. 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 always the realist and his suggestion here is that George should kill Lennie himself rather than let his friend fall into the hands of Curley or law enforcement. George ultimately takes Slim's advice. In the end of the novel Steinbeck implies that George and Slim will become close friends as they leave together from the same spot George and Lennie entered from at the beginning of the novel.

Whit mainly appears in chapter three. His main purpose is to help establish Steinbeck's theme of the importance of friendship and companionship. He shows Slim a magazine article which was written by a former worker at the ranch. Whit goes on about how he and the man were good friends. Whit also extols the virtues of Susy's whorehouse and tells George how he likes to go there on the weekends, further reinforcing the importance of companionship. Whit concludes, "Well, a guy got to have some fun sometime”. 

Carlson's main role is to help Steinbeck provide foreshadowing for George's killing of Lennie. Carlson suggests that Candy shoot his old dog, and when Candy hesitates Carlson offers to shoot the dog himself. In chapter three Carlson shoots the dog with his Luger, which is the same gun George will use on Lennie. George's decision is helped by the words from Candy: 

“I ought to of shot that dog myself, George. I shouldn’t ought to of let no stranger shoot my dog.”

The boss helps further establish the mental problems which face Lennie. He also represents the authority which always seems to be interfering with George's and Lennie's plans. Despite being warned by George repeatedly to not say anything when they talk to the Boss, Lennie can't help himself and repeats George's words in the boss's presence. The boss becomes suspicious and George becomes angry with Lennie for not remembering.

While George and Lennie should be considered the protagonists in the novel, Curley and his wife are antagonists. Curley is always belligerent and described as "pugnacious." Like his father he is an authority figure and partly responsible for George's and Lennie's ultimate failure in realizing their dreams. He fights Lennie in chapter three and is one reason why George decides to kill Lennie in chapter six.