"Episode 4 saw the words Bad Wolf appear for the first time. I just made it up on a whim, cos I liked the idea of the TARDIS being graffiti’d. But then I spent the rest of the episode idly wondering who that kid was, why he wrote those odd words. And, having dismissed notions of Evil Super Villain Kid, a plan began to form, in mid-production. Knowing that Rose would become the Time Goddess at the end of the series, I wondered if a Time Goddess would imprint herself on the universe, creating things in her image, like the face of Jesus in a bagel. Better still, these signs would actually summon her into existence. That’s the sort of thing you think about in this job, late at night. And then I worked backwards, inserting Bad Wolf references into almost every script. Funnily enough, I never told anyone what I was doing, in case it didn’t work, but the design department picked up on it—they didn’t even ask what it meant, they just offered to stencil it on Captain Jack’s bomb, in German. The idea spread without anyone knowing what it meant. Which is very Bad Wolf in itself."
Russell T Davies, Doctor Who: The Shooting Scripts (via timelordsandladies)