Right before Patrick reveals to the six-months-pregnant Mary that he plans on leaving her, he shows signs of anxiety. Dahl conveys Patrick’s nervous emotional state through imagery, as seen in the following passage:
"Listen," he said. "I've got something to tell you."
"What is it, darling? What's the matter?"
He had now become absolutely motionless, and he kept his head down so that the light from the lamp beside him fell across the upper part of his face, leaving the chin and mouth in shadow. She noticed there was a little muscle moving near the corner of his left eye.
Here, Dahl uses imagery to bring readers more closely into the scene and help them to see what Mary sees—how Patrick is eerily still, how “the light from the lamp […] fell across the upper part of his face,” how a “little muscle” is twitching “near the corner of his left eye.” All of these specific visual details help readers to picture Patrick in this moment and to understand that he is anxious and on the precipice of saying something that Mary may not want to hear.
This, of course, proves to be true, as Patrick goes on to reveal that he is going to be divorcing Mary, even though she is pregnant with their child. This act of betrayal is what leads Mary to kill him in a fit of rage and kicks off the rest of the story’s events.