Much of the humor in the story is provided by Mrs. Turpin's thoughts while in the doctor's waiting room. The other potential for comedy is writing a group of strangers together in a small space and watching how badly some of them end up behaving.
Mrs. Turpin is a shallow, small-minded woman, so her thoughts lend themselves to laughs. For example, we laugh at what Mrs. Turpin thinks is Jesus' sign of favor in not making her black or white trash:
Her heart rose. He had not made her a nigger or white-trash or ugly! He had made her herself and given her a little of everything. Jesus, thank you! she said. Thank you thank you!
Mrs. Turpin is ridiculous in her racial and class distinctions, which Jesus could care less about, and in not understanding that it is what you are like on the inside that counts.
It is also a comic moment when Mrs. Turpin states these same smug, self-satisfied thoughts out loud in an ecstatic outburst in the midst of the waiting room:
"If it's one thing I am," Mrs. Turpin said with feeling, "It's grateful. When I think who all I could have been besides myself and what all I got, a little of everything, and a good disposition besides, I just feel like shouting, 'Thank you, Jesus, for making everything the way it is!' It could have been different! ... Oh thank you, Jesus, Jesus, thank you!" she cried aloud.
It's darkly comic when at that point Mary Grace, the young woman Mrs. Turpin has labelled to herself as pitifully ugly and unpleasant, flings a book at her head and then:
the raw face came crashing across the table toward her, howling. The girl's fingers sank like clamps the soft flesh of her neck.
Mary Grace then says to her:
"Go back to hell where you came from, you old wart hog"
Mrs. Turpin remains oblivious to how annoying and infuriating she was in the waiting room with her smug self-righteousness and silly pronouncements, so much so that she was looked at by others as if she were an "idiot." She is resentful that she is not judged by her status in the class hierarchy:
She had been singled out for the message, though there was trash in the room to whom it might justly have been applied. The full force of this fact struck her only now. There was a woman there who was neglecting her own child but she had been overlooked. The message had been given to Ruby Turpin, a respectable, hardworking, church-going woman.
It is only at the end of the story that Mrs. Turpin's shallow sense of what is important, based on race, appearance, and "class," are challenged in a vision in which those "virtues" are burned away before she can enter the kingdom of God.