Since the characters of Ulrich von Gradwitz and Georg Znaeym, who begin the narrative of "The Interlopers" as mortal enemies apparently resolve their differences as they lie pinioned beneath the fallen mass of the beech tree, they must, then, be classified as dynamic characters. So, the reader is compelled to look beyond actual persons as the character who is flat/static. For, character can be defined, along with a person, as animals, things, or natural forces appearing in a literary work.
Clearly, Nature acts as the antagonist to von Gradwitz and Znaeym when their actions are foiled by the violence of the storm that "overwhelmed them both" and their intentions of shooting one another are quickly ended by the mass of falling beech tree limbs upon them. Nevertheless, it would seem that this accident of Nature has positive results as the two adversaries resolve their enmity, and, thus, Nature has changed to an emissary of friendship. But, this is not so. Nature remains the adversary as it attacks again in the form of wolves who answer the men's cries. Therefore, in Saki's short story, it is Nature that is the flat/static character.