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According to Russell T. Davies, the decision to establish the Bad Wolf meme in the series did not occur until after the spur-of-the-moment decision to have the words "bad wolf" graffiti-painted on the TARDIS; subsequently Bad Wolf references were added to the scripts for most of the other Series 1 episodes, and notwithstanding a few minor or inferred references in the interim, returned in force in the Series 4 episodes Turn Left (2008) and Journey's End (2008). With the origin of the meme established, the mystery that remains is exactly why the words "bad wolf" were chosen to be spray-painted on the TARDIS in the first place (as opposed to any other phrase).
When the Doctor complains of being slapped by Rose's mother, Rose laughingly remarks, "You're so gay!" This remark has caused some controversy in fan circles, some seeing it as an anti-homosexual slur. Russell T. Davies, who is gay, wrote in an e-mail response that it was the way people talked, and claimed that he was trying to provoke discussion by using the phrase.
The scene where The Doctor encounters the pig alien at the hospital was the first scene Christopher Eccleston filmed as The Doctor.
Naoko Mori would reprise her role of Toshiko Sato in Torchwood (2006). However, unlike her role as a pathologist in Aliens of London, Toshiko is portrayed as a technical expert on Torchwood. This continuity problem was fixed in Exit Wounds (2008) when she revealed that she was filling in for Torchwood's doctor Owen Harper, who was hungover.
Bad Wolf Reference: A young boy writes "Bad Wolf" in graffiti on the side of the TARDIS.
This is the first episode to examine deeply the impact which the departure of a companion with the Doctor can have on those left behind. In this case, Rose's family believed her to have been murdered, and her boyfriend Mickey had become a suspect. The impact of a companion's travels with the Doctor on family and friends back on Earth becomes a recurring theme throughout the Russell T. Davies era.