What is key to focus on is the various "triggers" that allow Walter Mitty to escape his humdrum life and his hideous and irritating wife and enter his world of fantasy where he is dominant, in control and superior. In fact, the story presents us with a number of ways in which the world of fantasy and the world of reality interact and even impinge on each other. Consider the first fantasy of Mitty's when he is the commander manages to get the hydroplane through the storm by accelerating, which is interrupted when Mitty's wife criticises him for going too fast in the car. Driving past a hospital triggers another fantasy of Mitty where is the doctor performing an operation that no-one else is able to do, which is interrupted when Mitty nearly crashes the car. A newsboy shouting about a trial allows him to enter a trial where he is the accused, then his insult to the District Attorney triggers the mundane focus on food for his dog. Finally, smoking and leaning against the wall of the drugstore triggers the fantasy of facing the firing squad.
Clearly all of these fantasies are an escape from a humdrum life of painful monotony - a cry for the need or desire to be important, to be spectacular and special from someone who is anything but and is controlled and dominated in his real life by his monopolising wife.
Walter seems to shift in and out of reality at least three times in this piece. To me, he tends to float into that world of illusion and imagination as triggered by his wife.
In the beginning he is piloting a Navy hydroplane that is in trouble. There seems to be a crew working around him that is dependent on his skills for their safety. He believes he is saving the day.
His wife rudely interrupts his dream and he is back to a dead-beat husband in no time with the chore of escorting his wife to get her hair done.
Before too long, he is in the world of imagination again and becomes a famous doctor tasked with operating on one of Roosevelt's closest millionaire friends.
Traffic in the parking lot captures the reality of his attention again and then before we know it, he is lost as a hot-shot lawyer.
After tackling the worst of the worst, Mitty is struck by the need to pick up his wife, after a bout back in the military, he gets her, and then the rain makes him feel as if he is in front of a firing squad.