What are the answers to the facts of the story in The Secret Life of Walter Mitty?
Walter Mitty is an ordinary man with an ordinary life who spends a good deal of his time daydreaming himself into adventurous situations where he is a hero. His imagines himself a brave pilot, a skilled surgeon, a gifted marksman, as someone who is facing a firing squad. All his waking dreams have the same theme, Walter Mitty is a powerful, sought after hero with extraordinary skills.
He spends so much time escaping his real existence, that he is constantly facing dangerous situations. When one of his fantasies kick in, Walter Mitty is instantly changed into the star of his own daydream, he feels the situation around him, therefore becoming less conscious of what he is really doing, like driving his car.
Walter Mitty is a man who is chained to a mundane existence. To survive the dullness of his regular life, he makes up scenarios that will make him feel successful. It is a wonder, though, why he doesn't apply some of his creativity to changing his actual existence.
In my opinion, I think Walter Mitty is an extreme case and that he borders on mental illness. You can easily see Walter Mitty being content sitting in some psych ward looking out the window, spending his days lost in his imagination and his fairy tales.