Much of the episode was filmed around the desert area of Almería, Spain, where studios have built Wild West-style streets that have been used in the making of over 100 Western-set films, such as The Dollars Trilogy.
The major change made to the episode was the elimination of a hauntingly beautiful melody emanated by Kahler-Tek, which typically presaged his arrival. Originally, Kahler-Jex eventually revealed that this was music he played during the torturous operation which created a cyborg, in order to drown out the subject's screams.
An early version saw the cyborg killed off partway through the story, which then focussed on the true villain of the piece, Kahler-Jex. This approach was soon inverted, with Kahler-Jex shot to death by Isaac's mad father George, and the drama of the final act generated by the cyborg seeking to punish the town that had deprived him of his revenge. This culminated in the Doctor confronting Kahler-Tec with the projected image of a woman named Kahler-San, who wore the same pendant as the cyborg. Although Kahler-Tec saw through the deception, it was enough to convince him to halt his assault on the people of Mercy.
The Wild West has not been a setting for a Doctor Who episode since the "The Gunfighters" in 1966. Toby Whithouse was advised not to watch The Gunfighters by the other writers, who said it was "not exactly the jewel in the crown"