Slim is an interesting character in the book. He is always described in a very positive way. He is oddly majestic in how he looks, moves, and behaves. This is why all the people on the ranch respect him. For example, the men look to him for approval. So, when the men wanted to shoot Candy's dog, the final word is given to Slim. Not only this, Curley, the boss's son, who thinks he has power over everyone, does not dare cross Slim. In short, Slim has authority on the ranch. Finally, Slim is the only one who understands George. Here is what the text says about Slim:
When he had finished combing his hair he moved into the room, and he moved with a majesty achieved only by royalty and master craftsmen. He was a jerkline skinner, the prince of the ranch, capable of driving ten, sixteen, even twenty mules with a single line to the leaders.
As for Carlson, he is one of the workers on the ranch. He is portrayed as mean and cruel. He is the one who suggested that Candy's dog to be shot. He also had a gun, which was used to kill Lennie. He is not a very interesting character in the book.