The Secret Life of Walter Mitty
Walter Mitty's characterization is defined by the author's text
Walter Mitty is in an unhealthy relationship. His wife is always yelling at him over unimportant things. Mrs. Mitty is always pushing Walter around and putting him down.
Walter isn't aware of his surroundings. He daydreams at inappropriate times. Also, when he daydreams, they effect his feelings and emotions.
Walter Mitty wishes for a different life. In his daydreams he is praised by others. In real life he is put down by others and he wishes he wasn't.
That is how Walter Mitty is characterized by the text of the author.