A few months ago, while playing Left 4 Dead, I found myself cowering in a closet, absolutely terrified to go back out and face the horde. I knew that I had to move so the game would continue, but I just didn't want to go out and take my chances. I didn't know at the time that L4D has this sinister AI called The Director, which keeps the game constantly changing depending on what the player does, adjusting things to keep the game fun. For example, if you're cowering in a closet and think you're safe there, The Director will send a bunch of the Infected to claw down the door and give you a friendly little nudge so you'll keep going.
Eventually, I was dragged out of the closet by a Smoker, saved by Francis, and went on to finish the level, heart pounding and my hands trembling just a little bit. It was awesome.
The next morning, while I ate breakfast, I was thinking about what it would be like if that was real. What would it be like if you really couldn't just cross the street any more, because there were zombies everywhere who wanted to kill you? What would it be like if you knew that, whatever you did, wherever you went, there would always be more zombies coming at you, night and day, until you died? In zombie stories, the characters are always heroic and noble (with rare exception, and even then we know those characters are just there so they can get their comeuppance) so I wondered what it would be like to write a story where the main character looks around and decides that sooner or later the zombies are coming in, the survivors are going out, and either way, they're totally fucked.
I wrote the whole story, doing my very best to ignore the various voices of doubt and such that keep coming back (yes, like zombies) no matter how many times I think I've killed them. It was really fun to write. I knew more or less what I wanted to do with it, and I sort of knew who the characters were, but I didn't stop myself from making things up as I went along, if something caught my eye and seemed worth exploring.
Yesterday, I finished the major rewrites, and though it still needs some editorial polish and Andrew's Red Pen Of Doom, I'm anxious to publish it with some other shorts I've worked on recently.