The bold contrast between day dream and reality helps to characterize Walter Mitty as a victim, as lacking self-esteem, and as being submissive
First, Walter Mitty is victimized by depression throughout the majority of the story. In the story, His only response to anything that anyone says is "Oh". Also, after the parking attendant takes his car to park it himself, Walter beings to walk around aimlessly.
Second, Walter Mitty lacks the self confidence to stand up for himself when others around him are unkind. When his wife tells him to put on his gloves, he does so, even though he does not want to. However, when his wife left, he took them off. Also, his wife tells him that he needs to see a doctor, but he doesn't stand up to tell his wife that he does not need to.
Lastly, Mitty is submissive and he does whatever it is that people tell him to do. in this story, Walter's wife tells him to buy overshoes, and even when he says that he does not need them, he stills goes to buy them. Plus, instead of telling people that he does not want to do something, while he does the orders, this is when he experiences his many day dreams.