in what way are Miss prism and chasuble stock characters?

Both characters are meant for amusement or farce. Miss Prim obviously loves Chasuble, though the fact that he is a priest prohibits her from telling him so directly while Chasuble frequently visits Jack's country house to see Miss Prism. Though he is celibate, he seems well matched for the educated Miss Prism. There is humor to be mined from this by keeping them  stock characters.