How does Walter show that he is a round character in the shift of scenes at the hospital in "The Secret Life of Walter Mitty" by James Thurber?

James Thurber's character Walter Mitty has become an archetypal character for the ineffectual, weak, bumbling, hen-pecked man in America.  As such, he is not a round character, for the only development that takes place in his personality is in his daydreams. In the hospital scene, for instance, Mitty imagines himself as the ingenious, heroic surgeon who "saves the day" and is rewarded for his superiority by being asked to perform a critical operation while the "crave figure of Benbow, who drank... and the the uncertain faces of the two great specialists" must defer to Mitty's genius.  However, once jarred out of this reverie by the mechanic's "Back it up, Mac!"  Mitty meek character returns, "Bee. Yeh."

Later, as he waits for his wife at the hotel in a winged armchair that shields him from view, his wife accosts him angrily,

I've been looking all over this hotel for you....Why do you have to hide in this old chair?  How did you expect me to find you?

Ineffectively, Walter attempts to assert himself:

I was thinking....Does it ever occur to you that I am sometimes thinking?

When Mrs. Mitty simply ignores his feelings by saying that she is going to take his temperature when they arrive home, Walter offers no dissent, proving that he is still submissive and weak. In the end, Mitty again retreats into his daydreams; his final one finds Mitty symbolically in front of a firing squad [his wife] where he is shot down, "Walter Mitty the Undefeated, inscrutable to the last"--the irony cannot be missed here as he is, in reality, defeated and predictable.