In yesterday's post, Davin's comment about multiple elements in stories made me consider how we go about targeting our primary audience. My response in the comments, while somewhat effective, doesn't get into the heart of the matter as well as I'd like.
As Davin states, someone might pick up your work even if they're not in your target audience. And though he is a scientist, we know that Davin prefers to write (and read) literary fiction. He also has a thing for Leo Tolstoy, but we won't get into that here ;).
However, this doesn't preclude him from say, picking up a science fiction novel, or a mystery, or an epic fantasy/thriller/insert genre here based on any number of factors. He could know the person (thank you for the compliment, by the way), get a recommendation, or even be entranced by the cover art.
Unlike public relations and marketing, where your audience is ironclad, audience in novels is a much more fluid thing. It's also infinitely more organic. I mentioned yesterday that I tend to write stuff for a more science-y crowd -- that's my audience because those are the types of characters I most often hear in my head.
Renee, Susan, and Anita, on the other hand, write for teenagers because (I'm assuming here, ladies, so correct me if I'm wrong) that's the age level they hear the voice of their main character in. Bane writes MG. Same reason.
So you see, your target audience changes based not on research and targeting and all that fun PR stuff, but on who the main character is. This is also why someone like Davin (or any of my dear readers) can pick up a random book in a different genre and still enjoy it. You might not understand the common tropes in the story, but if it's a good novel then it doesn't matter. An interesting plot or an interesting character will always win the day.