Our brains are wired so that if something doesn't fall into a category, then it's invisible. If we can't classify it, then we have a harder time making sense of it, so we ignore it. The last time someone recommended a particular author or band to you, what was the first question out of your mouth?

This helps us find what we like, but human beings largely use categories to filter out what they don't want. For example, the genres of "heavy metal", "new age", and "electronica" are tags that are applied to music that appeal to subcultures, but which are best avoided by most people. We know to walk right past the "Romance" section in the bookstore.

We needed to create the genre so the right people will find the contents, while the rest of us can ignore them.

Listening to people discuss movies currently playing in the theater reveals how this works. One person suggests that another go see a particular movie, and the reply comes back: "What kind of movie is it?" If the suggester has trouble shoehorning it into a particular genre, the next question is: "What other movies is it like?"

These two queries, about category and comparison, are so common that they almost guarantee that moviemakers aren't working hard to innovate very often. Venturing into truly innovative territory is extremely high-risk. If moviegoers are saying of your movie, "Look, it's hard to describe...just go see it, it's good", then 9 times out of 10, as a filmmaker, you're probably in trouble, since so few people are apt to risk paying to see a movie based on a vague description.

If you're smart and play it safe, then you'll pick the category before you start and work from there. If you're smart and want to take risks, you'll ignore the categories involved and create what you want despite them. Two kinds of smart (I just categorized them), each with belying different motives and each capable of achieving great things. But you probably should be aware of which one you are before you begin.