First, one needs a definition of indirect characterization. This is the kind of characterization inferred from the way other characters act in the story towards a specific person. The author gives some direct characterization at the beginning of the story, when he states that Mr. Nutell is at the manor for a rest cure.
Even in indirect characterization, one gets an idea that Mr. Nuttel is quite high-strung. Early in the story he receives letters of introduction "so that he will not mope." Also, Vera waits until they have "silent communion"; one gets an idea that something is going to happen to Mr. Nuttel at that point. Vera then goes on to tell Mr. Nutell this tragic story about three men going shooting in the marsh, and the poor man believes it entirely. Vera even leads him to believe it further when she calls his attention to the open window.
We learn much about Mr. Nuttel in an indirect way in the third paragraph from what his sister has to say. She fears that he will "bury" himself "down there" and not speak to anyone. He accepts her negativity without protest. She offers to give him letters of introduction because she feel he is to shy to introduce himself.
It is a good bet that the niece has been told something of Mr. Nuttel's condition because from the start, she tries to feel him out with questions like "Then you know practically nothing about my aunt?" Each answer that he gives her allows her to prey on his weakness.