The solo instrument that produces the melody in the title music is a theremin. It was invented by Russian scientist Leon Theremin, and was first demonstrated in 1920. It predates the modern synthesizer by about forty years, but its uniqueness stems from the fact that it is the only instrument that is played without actually being touched. Its electronic circuits are controlled by two antennas, left and right of the instrument, toward which the player moves his or her hands. The closer the right hand to one antenna, the higher the pitch. Similarly the proximity of the left hand to the other antenna controls the volume. The theremin has a range well in excess of eight octaves, and is capable of all kinds of strange effects. These sounds, perhaps most famously heard as the lead instrument of the long-running "Doctor Who" series, have also been put to use in other science fiction and movies, including Sir Alfred Hitchcock's Spellbound (1945) and Robert Wise's The Day the Earth Stood Still (1951). This unique instrument has also been used on The Beach Boys' song "Good Vibrations". The late Clara Rockmore was the theremin's greatest virtuoso, and the instrument and its inventor were profiled in the documentary Theremin: An Electronic Odyssey (1993).
Neil Dudgeon played gardener Daniel Bolt in season four, episode one, "Garden of Death", before coming back as Detective Chief Inspector John Barnaby in season thirteen, episode one, "The Sword of Guillaume".
The actors' and actresses' credits at the beginning of each show are rigorously egalitarian. After John Nettles or Neil Dudgeon and the actor playing the current Detective Sergeant, the actors and actresses playing the major supporting roles are listed alphabetically, no exceptions. As a result, well-known guest stars like Edward Fox, Honor Blackman, Hayley Mills, Claire Bloom, Peter Capaldi, Henry Cavill, Jenny Agutter, Orlando Bloom, Mark Gatiss, and Sir John Standing, amongst others, get no special billing. They're simply mixed in with the regular supporting cast and others.
Filming for an episode titled "The Blood Point" was abandoned after a few preliminary pick-up shots had been filmed because of unintentional close similarities between the plot and a real-life event.
The storyline in season sixteen about Sarah Barnaby's (Fiona Dolman's) pregnancy mirrored Dolman's real-life pregnancy.