The first and fifth episodes were written by Russell T Davies, who also wrote the story arc. The third episode was co-written by Davies and James Moran whilst the second and fourth were by newcomer John Fay, though, according to Moran's blog, all three writers had input into the storyline. Freema Agyeman and Noel Clarke were originally scheduled to appear as Martha Jones and Mickey Smith, Agyeman appearing throughout the story arc and Clarke in the final two episodes. When Agyeman was signed to co-star in Law & Order: UK, her appearance was whittled down to a one-scene cameo, which was ultimately cut when she became completely unavailable. Clarke was forced to pull out at a later stage due to film commitments. (REF: Doctor Who: The Writer's Tale - The Final Chapter) and Nicholas Briggs, who has written and acted in many licensed-by-BBC and unlicensed Doctor Who franchise spin-offs since the 1980s, and who is the revived Doctor Who series' resident alien voice-over artist, makes his first on-screen appearance in a televised Who-franchise production. An appearance by the Tenth Doctor was speculated on but did not occur, although references to the character are made in dialogue (including a reference in Day Five that serves to cover the issue of why he isn't on Earth to combat the 456).
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