Some writers will admit that they don't really think too much about the specifics of their story prior to writing it; neither its basic theme nor how it will play out. Many don't even bother to envision an ending. The story simply comes into existence as the protagonist lives through it. In this way, the writer acts as much chronicler as creator.
I have found that this technique—depending on your characters to show you the way—is a good method for coming up with story ideas as well. Think of a character you've written before, one you feel comfortable writing about without any set goal or storyline in mind. If you deposited this character in a random situation, do you know how he would act? If so, your knowledge of the character's personality and desires will drive his involvement in the scene to its logical conclusion.