Slim is different from the other workmen on the ranch. He is more intelligent, more observant, more tolerant and compassionate--altogether a superior sort of person. He takes an interest in Lennie, to whom he has given one of his dog's new puppies. In Chapter 3 he watches Lennie take his puppy back out to the barn and says to George, "He's jes' like a kid, ain't he."
George replies, "Sure he's jes' like a kid. There ain't no more harm in him than a kid neither, except he's so strong. I bet he won't come in here to sleep tonight. He'd sleep right alongside that box in the barn. Well--let 'im. He ain't doin' no harm out there."
This conversation excemplifies the growing friendship between George and Slim, both of whom are the most intelligent men on the work force. Unfortunately, both are mistaken about Lennie. It is ironic that George says that Lennie "ain't doin' no harm" out in the barn, because that is where Lennie will end up killing his puppy and then killing Curley's wife, leading to the tragic ending of the story.
The relationship created between George and Slim allows Steinbeck to use dialogue rather than exposition near the end of the story, when Curley's wife's body has been discovered and George is wondering what he should do about Lennie. George asks Slim:
"Couldn' we maybe bring him in an' they'll lock him up? He's nuts, Slim. He never done this to be mean."
Slim nodded, "We might," he said. "If we could keep Curley in, we might. 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."
Steinbeck was relying heavily on dialogue to tell his story because he had firm plans to adapt his novella into a stage play the same year the book was published. (See Introduction in eNotes Study Guide.) George's relationship with Slim enables him to explain his dilemma in the foregoing dialogue form. As readers we understand that George is doing his friend Lennie a service by shooting him and saving him from a lynching or a long, inhumane incarceration. There is no way that George could explain his motives to Lennie before he pulls the trigger. Slim confirms all of George's own thoughts and feelings and seems to be encouraging him to commit the mercy killing.
Slim is there at the very end and tells his friend George, "Come on, George. Me an' you'll go in an' get a drink. . . . I swear you hadda. Come on with me."
Slim is consistently helpful as a character because he enables Steinbeck to use dialogue to explain and dramatize what would otherwise be thoughts and emotions confined to the consciousness of George.