What effect does the corruption of innocence and the exploration of despair and fear in The Turn of the Screw have upon the reader?

I would want to argue that such Gothic conventions as you
have highlighted greatly enhance the feeling of terror that the reader experiences
through reading this excellent ghost story. Clearly, the way that Miles and Flora are
presented as innocents who have been corrupted by the evil Miss Jessel and Peter Quint,
though the nature of that corruption is left undisclosed, greatly enhances the shocking
nature of the story, whilst the appearances of the ghosts and the reactions they evoke
in the narrator cause us in turn to become terrified. Note the way in which the narrator
responds the second time she sees the ghost of Peter Quint in Chapter Four. The
suddenness of his appearance, and the picture of him standing on the other side of the
window and peering in, is terrible enough, but this is not
all:

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Something, however, happened this time that had
not happened before; his stare into my face, through the glass and across the room was
as deep and hard as then, but it quitted me for a moment during which I could still
watch it, see it fix successively several other things. On the spot there came ot me the
added shock of a certitude that it was not for me he had come there. He had come for
someone else.

The terrifying
appearance of this phantom is thus clearly linked to the way that he has come for a
purpose, and that purpose is obviously related to the children and his possession of
them or the malign influence that he has upon them. Connecting such a terrifying figure
to such innocent "angels" and the horror experienced by the narrator causes us, in turn,
to experience the same horror.