Chris and Billie really did a magnificent job with this scene. Just look at the facial expressions! He absolutely embodied the fear that The Doctor had for Rose's safety, and she completely nailed the fierce nature of the Bad Wolf. It was the two of them- along with a bit of brilliant script writing, of course- that made this scene so intense and perfect.
Like a transmitter. Round and massive, slap bang in the middle of London. A huge circular metal structure like a dish, like a wheel. Radial. Close to where we're standing. Must be completely invisible. What? What? Oh. Fantastic!