In discussing Steinbeck's direct and indirect characterization in his novella Of Mice and Men, I will focus on Carlson, since your question really should be expressed as two separate questions in accordance with the eNotes requirements.
It would seem that Steinbeck's choice of a title drawn from Robert Burns' poem was intended to apply, not just to George and Lennie, but to most of the characters. Candy did not plan to end up as a "swamper," and Crooks did not intend to end up in his degraded and precarious situation. Curley's wife wanted to be a movie star. Curley himself wanted a bride who would make him happy and not causing him anxiety and getting him into fights.
Carlson is directly characterized as a cold, taciturn, selfish man, but Steinbeck exhibits some sympathy for him through indirect characterization. Carlson's Luger is a very important object in the story. (The gun has to be a one-of-a-kind weapon because there is a question about who stole it until George takes it out of his pocket in chapter six. If it had been a Smith & Wesson .38 revolver, for example, it could not be instantly identified as Carlson's gun.) If Carlson was in his early twenties when he was discharged after World War I ended in 1918, then he would be in his early forties at the time Steinbeck's story takes place. Carlson probably had no idea that he would end up doing hard farm labor for fifty dollars a month, with a bunk and mealtimes where there was hardly enough food to go around. He is another character whose plans didn't work out.
Carlson obviously treasures that German Luger. It probably represents the one time in his life when he felt strong and brave and important, the one time when he felt he had a genuine purpose in life. He might have even been a noncommissioned officer. So many men were getting killed in the trench warfare that field promotions were an everyday occurrence. Chances are he took that Luger away from a German officer in his one moment of glory. He uses the pistol to fire only one shot--but this is how Steinbeck describes what he does with it after he has killed Candy's dog:
Carlson found a little cleaning rod in the bag and a can of oil. He laid them on his bed and then brought out the pistol, took out the magazine and snapped the loaded shell from the chamber. Then he fell to cleaning the barrel with the little rod. . . .Carlson finished the cleaning of the gun and put it in the bag and pushed the bag under his bunk.
The way he treasures that gun shows that it symbolizes a lot of things to him which he no longer possesses, including youth, great expectations, and courage. He is a man who was capable of better things than being a bindle-stiff doing back-breaking unskilled labor to sustain a bare existence. Maybe this is why the smell of Candy's old dog was so much more offensive to Carlson than it was to the other men. The smell was a constant reminder that he was getting old himself, that he was living in a bunkhouse with no wife, no family, no home, no privacy or dignity, sleeping on a "burlap sack of straw that was a mattress" and keeping his meager belongings in an apple box nailed to the wall "with the opening forward so that it made two shelves for the belongings of the occupant of that bunk."
The sensitive reader can understand why Carlson may seem sullen and surly, even somewhat cruel. He is a loner among loners, a man with no future and no hope.