When it comes to 99% of movies, genre is pretty black and white. Based on the story and the setting, there's a certain number of tropes and archetypes that are expected to feature, and even the most innovative examples tend to stick pretty close to at least a few conventions.
The more genres you try and add into the melting pot, the more difficult it becomes to balance them successfully , and in plenty of cases trying to hard to bring something new to the table can backfire and leave the whole thing as a mess. However, there have been more than a few titles over the years where it appears that the creative team simply decided to down tools halfway through, tear up the script and set the rest of the narrative in another genre entirely.
Admittedly, it doesn't always yield spectacular results, but when it works it gives the movie in question a completely new lease of life, throwing the audience for six as things take a drastic left turn into completely uncharted territory that nobody was expecting during the opening credits.
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