Director Fred Zinnemann wanted the Jackal to be played by someone anonymous and indistinct, so he eschewed famous names in favour of casting a then-unknown actor named Edward Fox. He later admitted this concept may have led to the film's below-expectations performance when it released in theatres.
The novel and film adaptations of "The Day of the Jackal" caused the London Public Records and Passport offices to tighten their regulations to avoid/reduce the chance of anybody stealing and using a deceased person's identity, as the Jackal carried out.
During the filming of the final sequence where President de Gaulle is presenting medals to veterans, the large crowd of extras were unaware of how close a resemblance actor Adrien Cayla-Legrand bore to the actual President. On the first take, when the President exits his limousine, most of the crowd gasped and an elderly extra, who was playing one of the veteran soldiers, fainted in shock.
When the Jackal looks in the hotel registrar to find out which room Madame de Montpelier is staying in, the names of the production crew for France can be seen: assistant director Louis Pitzele, chief grip René Strasser and set designer Willy Holt.
When the British find the bogus passport request it's mentioned that it was applied for on July 14. That is Bastille Day, a national holiday in France that commemorates the storming of the French prison the Bastille (which signified the start of the French Revolution the end of the Monarchy in France).
Presumably to avoid confusion with the names of living French people, the fictitious names used by Frederick Forsyth in the novel were replaced by those of famous French historical people: for example, Colbert (Louis XIV's Minister of Marine), Dumont (famous explorer), Berthier (Napoleon Bonaparte's chief of staff).
The first scene filmed was the Jackal's meeting with the OAS members. It took three days to shoot, as Edward Fox was having a hard time getting into the character. It took a long drive with Fred Zinnemann, who assured him it wouldn't harm his career if he didn't get it right since he had been Oscar-nominated, before he finally got the portrayal right.
Casson mentions the Jackal having done "that fellow in the Congo". He probably refers to Patrick Lumumba, the first Congolese president, who was rumored to have been assassinated by a European; this also alludes to Frederick Forsyth's third novel, 'The Dogs of War', which focused on European soldiers (mostly mercenaries) getting involved with African affairs (conflicts commonly) back in the 1960s-70s.
Earlier in the film, Colbert exclaims how 960000 Francs got stolen in three weeks by the OAS in a series of robberies. The scene was set on 6th July 1963, 3 weeks after the Jackal met the three OAS men. As in July 1963, the exchange rate between French Francs to the US Dollar was 4.9371 : 1, which when converted stands at $194,447, short of the $250,000 advance money to be paid to the Jackal.
There are 31 individual insert shots of clocks in the movie. By contrast, High Noon (1952), also directed by Fred Zinnemann and more directly concerned with the passage of time, contains only 13 insert shots of clocks.
When the Jackal meets the weapons supplier in Genoa, there is a picture of John F. Kennedy on the cover of an Italian magazine reporting on President Kennedy's recent visit to Europe. The scene is set on 2 August 1963, about three months before Kennedy himself was assassinated.
One of the Jackal's targets was a man named Trujillo. This is likely Rafael Trujillo, the president of the Dominican Republic, who was considered a tyrant and a dictator and who was assassinated in 1961.
The Liberation Day scenes were filmed at a real parade, with most spectators being unaware of a film being shot. This caused a bit of confusion: many of the crowd mistook the arrests being filmed for real ones, and attempted to assist.
The trivia items below may give away important plot points.
A scene was filmed where the Jackal shoots the Gunsmith with his custom-made gun (right after the Jackal asks for one explosive bullet from the Gunsmith). This scene was cut so that it would lead smoothly into the Jackal's rifle test in the forest, as well as provide a more interesting dynamic with the previous scenes of the Forger (who was trying to blackmail him and got his neck broken for it).
A real-life forger told Frederick Forsyth three possible ways a person can obtain a false identity: apply in false name, steal another man's identity, or bribe an official for an "en blanc" passport and fill in the details. In the film, the Jackal uses all three, but the first was the most effective as it was very straightforward and very simple.