The way that I would want to approach this question would be through discussing the presentation of Mrs. Drover in terms of the appearance that she gives, but also looking at the tantalising hints of reality about her that suggest she is actually a very unstable individual who could be hallucinating the demon lover and the event that transpires in this story.
The setting seems normal enough: we are told that Mrs. Drover is returning to her house during the blitz in the Second World War to pick up some things. She is a middle-aged married woman who appears to be sensible and practical. This would be her appearance. Yet the text gives us enough evidence to suggest that the reality is very different. Note the reference to her general state of anxiety and her being "perplexed." Also, note the following description about her:
Mrs. Drover's most normal expression was one of controlled worry, but of assent. Since the birth of the third of her little boys, attended by a quite serious illness, she had had an intermittent muscular flicker to the left of her mouth, but in spite of this she could always sustain a manner that was at once energetic and calm.
Such details suggest a deep level of emotional instability that is covered up by the carefully controlled "manner" that she shows to those around her. This would indicate that there is something much more disturbing going on in Mrs. Drover than we would first believe, and begs the question about the reality of the demon lover that haunts her.