When Martha has returned to the TARDIS and is watching the video the Doctor made for her she fast forwards through a long section before reaching the Doctor's last point. There was no actual script for what David Tennant was saying in this section, as we see in the deleted scenes. Instead he lets loose a babble that covers not only the fact that he has to talk for a minute and a half, but he also tells us about The Housemartins and a gig of theirs he saw in 1990, before wrapping up with a string of random words and sounds.
The aliens in this episode were intended to have technology that was very organically based and this carried through to the design of their guns. The guns' theoretical design was that there was a creature inside each gun and pulling the trigger resulted in a jab to the creature, who would then scream; this scream is the disintegration beam.
This episode and part two "The Family of Blood" were adapted by scriptwriter Cornell from his 1995 Doctor Who novel, "Human Nature." This marks the first time in Doctor Who history that a novel based upon the series was adapted as an episode. Many changes were made in transferring the novel to the screen. The original book featured the Seventh Doctor and his companion, Benny Sommerfield, and the Doctor's motive for becoming human was to study the human condition, not to flee aliens.
In the opening of the episode, The Doctor tells Martha Jones that the Family of Blood is able to follow them even though they are fleeing in the TARDIS because the Family has stolen a Time Agent's Vortex Manipulator. This suggests that the Family have, in the past, encountered one of the five Time Agents aside from Captain Jack Harkness and Captain John Hart, as both are still in possession of theirs.
John Smith's birthplace was given as Gallifrey. This planet The Doctor is from. Though in this episode he states it is somewhere in Ireland. He says he was brought up in Nottingham to reflect David Tennant's desire to continue using his English accent.
The Aubertides of the novel were creatures whose entire being (including their clothes and possessions) was an extension of their form. Paul Cornell tried to make this work on screen, including giving the little girl (whose last name was initially Wainwright, changed to Cartwright because Blink already featured a character with that surname) a sentient, attacking balloon, and having Martha realise that Jenny is an alien when her friend reacts with pain after Martha spills tea on her handbag. Ultimately, however, it was decided that this notion worked better in prose than as television, and so the Aubertides were replaced by the Family of Blood.
Fearing that a straight adaptation of Human Nature would be too old-fashioned for Doctor Who (2005)'s modern storytelling sensibilities, Paul Cornell struggled mightily with the construction of his scripts. At one point, for instance, the adventure opened with the Doctor already married to Joan Redfern (now the school's nurse rather than a teacher). Russell T. Davies encouraged Cornell to hew more closely to his novel, however. Later, Cornell had the timeline for the episodes stretch over several weeks to provide time for John Smith's courtship of Joan to play out, but it was found that this removed too much intensity from the Family's pursuit of the Doctor.
The setting was shifted forward to the summer of 1914, and then back to the winter of 1913 in order to ensure that viewers not confused as to when the events of the story fell relative to the start of World War I.
The trivia item below may give away important plot points.
The fob watch that figures so prominently in this episode (and through the rest of the third series) was originally seen in the early series one episodes as a knob on the TARDIS control console. It is clearly seen in closeups of Christopher Eccleston's hands as he sets the controls in Doctor Who: The End of the World (2005).