In the aftermath of the nuclear blast, footage of a cat supposedly "suffocating" outside in the extreme heat is shown. This is actually footage of a cat enjoying a hefty dose of catnip, then they just reversed the film to give the impression of the cat suffocating (the way the cat is rolling on the ground is the giveaway).
While shooting the movie, the BBC got in trouble with local police when they detonated a large smoke bomb in order to simulate a nuclear explosion. Members of the public, who were not aware that a movie was being shot, panicked and thought that a real explosion had occurred.
Findings from the 1980 British Government exercise "Square Leg" were used as the basis for projecting the level of destruction and number of casualties in the movie. "Square Leg" was a government project that estimated what would happen in Britain in the event of an actual attack. It projected the mortality rate at 29 million, serious injuries at 7 million and short term survivors at 19 million.
Mick Jackson subsequently travelled around the UK and the US, consulting leading scientists, psychologists, doctors, defence specialists and strategic experts in order to create the most realistic depiction of nuclear war possible for his next film.
Mick Jackson consulted various sources in his research, including the 1983 Science article Nuclear Winter: Global Consequences of Multiple Nuclear Explosions, penned by Carl Sagan and James Pollack. Details of a possible attack scenario and the extent of the damage were derived from Doomsday: A Nuclear Attack on the United Kingdom (1983), while the ineffective post-war plans of the UK government came from Duncan Campbell's 1982 exposé War Plan UK.
Mick Jackson later recalled that while BBC productions would usually be followed by phone calls of congratulations from friends or colleagues immediately after airing, no such calls came after the first screening of Threads. Jackson later "realised... that people had just sat there thinking about it, in many cases not sleeping or being able to talk." He later said that he had it on good authority that Ronald Reagan watched the film when it aired in the US. Hines himself received a letter of praise from Labour leader Neil Kinnock.
It was repeated on BBC One on 1 August 1985 as part of a week of programmes marking the fortieth anniversary of the atomic bombings of Hiroshima and Nagasaki, which also saw the first television screening of The War Game (1965).
As part of their research, Mick Jackson and Barry Hines spent a week at the Home Office training centre for 'official survivors' in Easingwold which, according to Hines, showed just 'how disorganised post-war reconstruction would be'.
Mick Jackson hired Barry Hines to write the script because of his political awareness. The relationship between the two was strained on several occasions, as Hines spent much of his time on set, and apparently disliked Jackson on account of his middle class upbringing.
The scenes taking place six weeks after the attack were shot in the Peak District National Park, though because weather conditions were considered too fine to pass off as a nuclear winter, stage snow had to be spread around the rocks and heather, and cameramen installed light filters on their equipment to block out the sunlight.
The film was first commissioned by the Director-General of the BBC Alasdair Milne, after he watched The War Game (1965), which had not been shown on the BBC when it was made, due to pressure from the Wilson government, although it had had a limited release in cinemas.
Mick Jackson initially considered casting actors from Coronation Street (1960), he later decided to take a neorealist approach, and opted to cast relatively unknown actors in order to heighten the film's impact through the use of characters the audience could relate to.