Slim's comments about people, in general, are reinforced through the first encounter between Curley, George, and Lennie. The opening confrontation in chapter 2 is one in which Curley focuses on Lennie talking because he is so much bigger than Curley. Curley comes across as a brute and bully to reinforce his position of power on the farm. Curley also does this to clearly demonstrate that he is not afraid of Lennie, even through Lennie is larger than he is.
When Slim first appears in the latter half of the chapter, his comments speak to this opening encounter between the three men. Slim comments about how George and Lennie "travel around together." When he speaks to this a bit more, Steinbeck makes it clear that Slim "looked through George and beyond him:"
“Ain’t many guys travel around together,” he mused. “I don’t know why. Maybe ever’body in the whole damn world is scared of each other.”
For Slim, these comments speak to the fundamental encounter that George, Lennie, and Curley had in the earlier part of the chapter. Curley is clearly scared of the world around him. In compensation of this fear, he seeks to antagonize other people and seeks to fight with others. This is the root of his problem with Lennie.
At the same time, Slim's comments about Lennie in chapter three are also telling regarding the initial encounter. Slim and George speak of Lennie and Slim comments about how Lennie is a "nice fella:"
“Guy don’t need no sense to be a nice fella. Seems to me sometimes it jus’ works the other way around. Take a real smart guy and he ain’t hardly ever a nice fella.”
These comments speak to how Lennie is nice enough to not recognize that he is being targeted by Curley in the initial encounter. At the same time, Slim's comments speak to how much meanness is within Curley. Slim's distinction between "smart" and "nice" is telling in that it helps to define how Lennie interacts with the world around him. In these comments, one sees how Slim helps to articulate the condition in which Lennie lives and will ultimately suffer as a result.