A Plot twist is a change ("twist") in the direction or expected outcome of the plot of a film or novel. It is a common practice used to keep the interest of the audience, usually surprising them with a revelation. Some twists can be predicted and are preempted by numerous hints whereas others are a complete shock - most are somewhere in the middle with some people foreseeing them and others not.
Revealing the existence of a plot twist may be enough to spoil a movie for some, as the time spent watching would be used to "search" for the twist itself, as opposed to simply enjoying the presentation.
In films, M. Night Shyamalan is well known for having a substantial twist towards the end of each of the films he has written and directed.
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