In Stephen King’s memoir, On Writing (which is excellent; I highly recommend it), he talks about his “Ideal Reader,” which is the one person you write for. In King’s case, it’s his wife Tabitha. When you’re writing your first draft, you think of how your ideal reader would react to certain scenes or characters. Keeping this person in mind can keep you from straying down nonsensical plot lines and paths of ridiculousness. The ideal reader can simultaneously be your cheerleader and your critic while you’re writing the first draft (or the first few drafts).
I don’t necessarily think that this “ideal reader” has to be the same for everything you write. You may write one story with your best friend in mind, and then you might write another with your significant other in mind. Or maybe your ideal reader is actually a group of people: like three of your friends, or the members of your church, or teenage girls with broken hearts… you get the picture.
Most of the time, my boyfriend is my ideal reader (happy anniversary, babe!). But there are certain things I’ve written that I’m pretty sure he wouldn’t be interested in, like THE ARCHIVES, which I mainly wrote for a female audience. Think about someone who you think will like the book once it’s published… a person who will be the book’s advocate.
The concept of an “ideal reader” also makes me think of writing with an “agenda” in mind. If you’re writing a book about politics, your ideal reader might be someone you’re trying to convince with your agenda. To me, converting someone with a hidden agenda isn’t the best reason to write a book (especially if it’s fiction), but even so… that could be a type of ideal reader.
When the book actually gets published, its actual audience is very different from the ideal reader. Obviously, many more people read Stephen King’s books than just his wife. Those people are going to have an entirely different take on the book than his wife did when the book was in its draft stages. So the ideal reader is sort of like a beta tester… imagining them reading the draft (or actually reading the draft) might keep you, as the writer, from introducing elements that might put off the book’s eventual audience.