I would argue that the relationship of the governess with other members of the household, in particular with Mrs. Grose, the rather naive and innocent housekeeper, is key in the way that the plot develops and eventually resolves itself. Mrs. Grose, from her first introduction, is described as being a rather innocent and stupid lady, who therefore shows that she will believe everything that the governess, a much more intelligent woman, says to her. It is clear that the governess uses Mrs. Grose as a collaborator in the way that she reads what she thinks is happening, and Mrs. Grose's confidence gives her the necessary information she feels she needs to act on her fear of Peter Quint and Miss Jessel. If you look at the beginning of Chapter Six, it is clear that they are united together in this situation, at least from the perspective of the governess:
It took of course more than that particular passage to place us together in the presence of what we had now to live with as we could--my dreadful liability to impressions of the order so vividly exemplified, and my companion's knowledge, henceforth--a knowledge half consternation and half compassion--of that liability.
After this passage, Mrs. Grose accepts the truth of the words of the governess and, in addition, treats her as having the "questionable privilege" of being able to see things that she cannot. It is thus the complicity between them, and the way that Mrs. Grose aids the governess, that helps to a large extent to create the fascinating ending of this great ghost story.