The Doctor (Doctor Who) footage. Tom Baker did reappear, but as "the Curator", an ambiguously different character who he was not credited for playing. A voice actor, John Guilor, recorded a line of audio impersonating the First Doctor, for which he was credited as "Voice Over Artist". In other multi-Doctor stories, the multiple actors are all credited as "The Doctor", the exception being "The Three Doctors" (1972–73), which credited William Hartnell, Patrick Troughton and Jon Pertwee as "Doctor Who" as the 1972 serial preceded the practice of crediting the character as "The Doctor". In "Human Nature" (2007), the plot involves the Tenth Doctor altering
The Lodger (Doctor Who) of the upstairs apartment. When Roberts began writing for the episode, he knew the series' overarching plot but was not aware who was to be cast as the Eleventh Doctor. Roberts based the Doctor's lines on those written in Moffat's completed scripts and further characterization was added by Matt Smith's reading of the lines. The episode also contains several cultural references. When the Doctor is having a shower, he is heard singing ""La donna è mobile"", which his third incarnation sang in "Inferno". When the Doctor introduces himself to the time ship's Avatar, he claims to be "Captain Troy Handsome