One of the way in which to turn a story in an unexpected direction ... is to have your protagonists sense of social reality challenged.
There are countless things in our daily lives that only truly exist, because we consider, as a society, that they do exist. Marriage is one, money is another. These things exist because we want them to, and we allow our lives to be governed by them.
There are other considerations that we have that are common, but not universal. If I consider the streets are safe at night because of past experience, and common knowledge in the area, then I may believe it.
When those things that are hallmarks of what is 'normal' are challenged, the results can be unpredictable, and affect notions up to that point in the novel that we have taken for granted.
This can be most dramatic when it occurs unexpectedly. I've been witness to a murder? My sense of mortality / religion / safety on the streets is threatened. All of a sudden, I am dealing with a new reality, a world in which the predictable has become alien and unknown.
The reason this device works so well, is that it's something we as an audience, all fear. We hold on to these commonalities, the social realities that maintain our sense of the world. When they are challenged, it's frightening ... and exciting!
