Trespass and faux pas
The stories have similar narrative mechanisms; they tend to involve depictions of faux pas or trespass. In "The Interlopers," the dissonance is fairly equal, but in "Down Pens," the advantage is given to the public and social conformity. They agree at the end of that short story that even though they can see right through a meaningless and tedious social convention, they still out to write the letters. Of course "The Mouse" is defined by faux pas, because Theodoric believes he is imposing his nakedness when in fact he is not.
Nakedness
When Janetta and Egbert discuss the practice of writing Thank You letters for every gift they receive, they come to the conclusion that the whole practice is ironically counter-intuitive and wrong. By over-celebrating the gift-giver, they rob them of the sincere joy of giving with nothing in response. They also discuss emotional nakedness, claiming that it would be better to do relationships without social pretense. That is the same idea being manifested in Theodoric Voler's story. He strips naked and confesses that he was simply trying to get a mouse out of his clothes, just to find out that the woman he is with is blind.
Animal life
In "The Interlopers," and "The Mouse," humans are made subject to fate by the introduction of animal life. This imagery of animal involvement in human affairs is a reminder that the humans are themselves part of this imagery. The humans in the stories are just like the mice that crawls up that guy's leg—his panic betrays his instinctual response to rodents. The humans are also just like the wolves who are the doom of the "Interlopers." The wolves are imagery pointing toward the vicious and destructive ways of animal life, as the human characters also do in that story.
Social convention
The primary imagery of this collection is social convention and the ways of human community. The stories have this in common because they all tend to tie thematically back to the idea that one has to sacrifice for the benefit of being in community. This sacrifice is measured out in various ways, and in "The Interloper," the distaste for that sacrifice is depicted at full volume. Instead of living peacefully, two neighbors scream and accuse each other. The story-teller's commentary? Well, the land isn't theirs in the first place, and besides—they are both at risk of death by wolf pack! Better to cooperate, it seems.