When Jack kisses Rose and the Doctor goodbye, John Barrowman made a special effort to kiss both actors in exactly the same way... except on one take, when he kissed Billie Piper as usual, then said his line to Christopher Eccleston, started the kiss, and didn't stop until they fell on the floor. (Source: John Barrowman on the DVD commentary)
The street corner at which the TARDIS reappears after being sent back to 2005 is the same street corner where Rose's father was originally struck by a car and killed in the opening of Doctor Who: Father's Day (2005) prior to Rose altering the circumstances of his death.
David Tennant's portion of the regeneration scene was actually filmed much later than Christopher Eccleston's, and without the presence of Billie Piper. Tennant's segment was recorded with him speaking to a piece of sticky tape indicating Piper's eyeline and then edited into the broadcast version.
Russell T. Davies wrote an alternative climax in which the Doctor and Rose actually head to the planet Barcelona, with Rose unknowingly on the verge of death due to her exposure to the time vortex. It was planned that this ending would still be recorded, at least partly to serve as a red herring. This ending has not been released to the public.
Initially, the Controller survived into this episode to provide the Doctor with someone to talk to, until Russell T. Davies decided it would be more effective to have him converse with the Emperor Dalek instead.
In an acting master class at the Theatre Royal Haymarket in July 2011, Christopher Eccleston expanded the reasons for his leaving the show. He cited creative differences with "the senior people" on the production and stated he did not like "the culture" of working on the production.
This was the first episode in this series which was not given a press screening prior to the broadcast. Radio Times stated, "No preview tape was available for this episode." The episode was, however, screened for BAFTA on 15 June 2005.
Rose's actions create a predestination paradox. The words "Bad Wolf" tell her to try to get back to the Doctor, and her doing so gives her the ability to leave the words through time as messages to herself, which she then does. Although it can be argued that the phrase "Bad Wolf" originates with the Badwolf Corporation, it can also be argued that she somehow prompted the creation of the phrase through her powers in the first place, thereby also introducing an ontological paradox. Ontological paradoxes were explored in Doctor Who: Blink (2007), where the Doctor explains that space-time is not strictly cause-to-effect, and serve as a major plot device in Doctor Who: Time Crash (2007). The Doctor himself moves in a fictitious five-dimensional setting (The Space Museum, 1964), and perhaps a six-dimensional setting (Inferno, 1970).
Jack kisses the Doctor affectionately on the mouth (after kissing Rose) before going off to fight the Daleks. This is the first same-sex kiss featured in the Doctor Who franchise. Another occasion could be that of Madame Vastra and Jenny in Doctor Who: Deep Breath (2014), although this was more so out of survival than affection.
The BBC produced a documentary entitled Doctor Who: 30 Years in the Tardis (1993), which ended with several scenes showing how modern-day special effects could be applied to "new" Doctor Who production. One of these scenes shows many Daleks hovering together. Whether by accident or intent, this episode includes several scenes that strongly resemble this "what if" scenario.
The only characters who do not die in this episode are Rose, Jackie and Mickey, who are all either main or recurring characters. Every guest character dies, along with the Ninth Doctor and Jack, though the latter is revived shortly afterwards. As a result, this is the first episode to kill off the entire guest cast since Horror of Fang Rock 18 years before, an act that would not be repeated again until Doctor Who: The Doctor's Wife (2011).
The Ninth Doctor's pre-regeneration statement, "I mean it's a bit dodgy this process, you never know what you're gonna end up with", echoes the Fifth Doctor's post-regeneration statement in Doctor Who: Castrovalva: Part One (1982), "That's the trouble with regeneration, you never quite know what you're going to get".
This is the first known time that the Doctor's kisses have had an effect on humans beyond the act itself. In this case, the Doctor removes the time vortex energy from Rose by kissing her. In the next, he imparts traces of his alien DNA to Martha Jones. (_Smith and Jones_) The Doctor and Donna Noble later kiss, but in that instance the Doctor is the one affected, as the kiss was a catalyst for an antidote to poison - Donna kissed him to give his system a shock. (Doctor Who: The Unicorn and the Wasp (2008)).
Series 1 contained an underlying story arc about the meaning and identity of Bad Wolf, with each episode containing several references to the name. In this episode, Rose sees dozens of Bad Wolf graffiti, which lead her to realizing that she is the Bad Wolf, and uses the words to always lead her back to The Doctor.