Since there are only two characters in this story, and since they are very different in so many ways, it would seem that a comparison and contrast between Walter Mitty and his wife would be an excellent topic for an essay.
Walter Mitty is an introvert. His wife is an extrovert. This combination is not unusual in marriages. Either the husband or the wife takes care of practical matters, including social activities. It is often the husband who is introspective and the wife who takes over the planning and the doing. This can work out successfully for both partners, as C.G. Jung states in his book Psychological Types.
It actually has worked out for Mr. and Mrs. Mitty. They seem to be permanently married. She complains about him but is not thinking of getting a divorce because she apparently enjoys mothering him. He is an extreme introvert and needs somebody to look after him, to pick out his clothes and tell him what to wear. He escapes from her in his imagination but apparently will never decide to escape from her through divorce.
Mitty has a vivid imagination. His wife has no imagination at all. Mitty is quiet. His wife is vociferous. They must have things in common which can be identified in a compare and contrast essay; otherwise they wouldn't stay together. They don't have any children but they do have a puppy. They must both like dogs. They both seem to enjoy domesticity, comfort, routine, respectability, security--that sort of thing. They don't fight or argue, as some of Thurber's other married couples do. (For example, "The Breaking Up of the Winships" and "A Couple of Hamburgers," both reprinted in Thurber's My World and Welcome to It and in The Thurber Carnival.)
Only once in the story does Mitty show any rebelliousness.
"I was thinking," said Walter Mitty. "Does it ever occur to you that I am sometimes thinking?" She looked at him. "I'm going to take your temperature when I get you home," she said.