Co-screenwriter Bruce Robinson claimed that the screenplay for the film was radically altered to exclude the story element that had interested him most about the film's subject matter. This was his controversial belief (which remains unproven) that the Jean Tatlock character, played by Natasha Richardson, had not committed suicide, as has always been reported (and as was depicted in the final film), but had, in fact, been murdered.
"Little Boy", according to the Atomic Heritage Foundation, was a "...gun-type device, the critical mass is achieved when a uranium projectile which is sub-critical is fired through a gun barrel at a uranium target which is also sub-critical. The resulting uranium mass comprised of both projectile and target becomes critical and the chain reaction begins. Dropped on the Japanese city of Hiroshima on August 6, 1945, it was the first nuclear weapon used in a war".
"Fat Man", according to the Atomic Heritage Foundation, "was the second plutonium, implosion-type bomb. The first was the 'Gadget' detonated at the Trinity site on July 16, 1945. In the implosion-type device, a core of sub-critical plutonium is surrounded by several thousand pounds of high-explosive designed in such a way that the explosive force of the HE is directed inwards thereby crushing the plutonium core into a super-critical state. Dropped on the Japanese city of Nagasaki on August 9, 1945, it was the second nuclear weapon used in a war".
The character Michael Merriman (John Cusack) was an amalgam of real life persons Louis Slotin and Harry K. Daghlian. Website Wikipedia states, according to Deborah J. Kunk in "'Fat Man' Brings Bomb Alive", that this character "is a fictional composite of several people and is put into the film to provide a moral compass as the 'common man'."
The Los Alamos laboratory and research facility center seen in the film is now known as the Los Alamos National Laboratory and is also known in abbreviate form by the acronym LANL. Other earlier names for the lab over the years have been Project Y, the Los Alamos Laboratory and the Los Alamos Scientific Laboratory.
The nick-name of the experiment where a radioactive element was dropped in a critical accident was "Tickling the Dragon's Tail". In his novel "Los Alamos" (1997), Joseph Kanon, the book's author and source novelist for the later movie The Good German (2006), recounted the circumstances surrounding the first critical accident, calling it "Tickling the tail of the sleeping dragon".
"Demon Core", according to website Wikipedia, "...was a 6.2-kilogram (14 lb), 3.5-inch-diameter (89 mm) subcritical mass of plutonium which went briefly super-critical in two separate accidents at the Los Alamos laboratory in 1945 and 1946. Each incident resulted in the acute radiation poisoning and subsequent death of a scientist. After these incidents the spherical plutonium pit was referred to as the 'demon core'. The demon core was used in the first atomic bomb test to be conducted after World War II, five weeks after the second fatal accident. It performed normally and with the same explosive yield as the next core used in this set of two tests".
The geographical location of "The Trinity Site" was in the Alamogordo Desert in south-central New Mexico, USA. This motion picture though was instead predominantly filmed in the middle-American more southern country of Mexico.
The Los Alamos Research facility and laboratory center seen in the film was a set construction recreation built especially for the picture located in the environs of Durango, Mexico. The gigantic set involved in 1988 the building of thirty-five dwellings with a budget cost of around US $2 million.
The meaning of the films "Fat Man and Little Boy" title, as outlined by show-business trade-paper 'Variety', is that "'Fat Man' and 'Little Boy' were the [code names or] nicknames given to the [atom] bombs dropped over Hiroshima and Nagasaki [in Japan in August 1945 during World War II]".
Some movie posters for the film featured a long text preamble that read: "In 1942, the United States secretly set out to build the world's first atomic bombs. They called them 'Fat Man' and 'Little Boy.' And the man they called on to get the job done was General Leslie Groves. Strong. Brilliant. Determined. Willing to push human endurance to the limit. Willing to bear the responsibility, the glory and the blame. His mission changed the world. His story is the story of his times. And ours."
Second and final [to date, June 2015] of two collaborations of director Roland Joffé and screenwriter Bruce Robinson who had both previously collaborated on The Killing Fields (1984) around five years earlier. Both pictures were war movies with the only differences between the pair's billing on both films was that on Fat Man and Little Boy (1989) Joffe was a co-screenwriter and Robinson was credited also for story.
"The Manhattan Project", as defined by website Wikipedia, "...was a research and development project that produced the first atomic bombs during World War II. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the U.S. Army Corps of Engineers [played in the film by Paul Newman]; physicist J. Robert Oppenheimer [portrayed in the movie by Dwight Schultz] was the director of the Los Alamos National Laboratory that designed the actual bombs".
The trivia item below may give away important plot points.
John Cusack's character, Michael Merriman, is a fictional composite, based primarily on Los Alamos physicists Louis P. Slotin and and to a lesser degree on Harry K. Daghlian. In particular, Merriman's incident with the plutonium hemispheres, which was a risky procedure known as "tickling the dragon's tail", is based on two real life accidents that resulted in the deaths of the scientists. Respectively, Daghlian died on 21 August 1945, less than a week after Japan's surrender, whilst Slotin died on 21 May 1946, after the Second World War had ended. Slotin had participated in the team that assembled the Trinity device. Both incidents involved the same plutonium core, later nicknamed the "Demon Core". This core was used in an atomic test at Bikini Atoll on 1 July 1946.