Remember the scene where the Doctor, Rose and Harriet Jones MP are locked inside the cabinet room, then the Doctor opens the blast doors to talk to "Margaret" Slitheen? It's the scene that ends with (roughly) the following:
DOCTOR: I'm gona stop you.
MARGARET: What, you? In your box?
DOCTOR: Yes.
Well, when they were recording Annette Badland's side of that exchange, Eccleston wasn't available. So Russell T Davies stood in for him. The intensity of Badland's performance gave him such a case of the willies that he resolved then and there to bring the character back. Hence, "Boomtown".
(RTD recounts this tale in the Series 1 Shooting Scripts book.)
no subject
DOCTOR: I'm gona stop you.
MARGARET: What, you? In your box?
DOCTOR: Yes.
Well, when they were recording Annette Badland's side of that exchange, Eccleston wasn't available. So Russell T Davies stood in for him. The intensity of Badland's performance gave him such a case of the willies that he resolved then and there to bring the character back. Hence, "Boomtown".
(RTD recounts this tale in the Series 1 Shooting Scripts book.)