The reason why George Clooney's character chews gum a lot in the film is because the character simply doesn't have a lot of dialogue and Anton Corbijn wanted Clooney's face to not be completely static throughout. This is also another reference to spaghetti Westerns as Clint Eastwood - also a strong, silent character - would often be chewing tobacco.
The American's riverside idyllic spot was much more barren in real life so the production added lots of extra plants to make it perfect. Guards had to be posted by the location at night to prevent it from being eaten by wild boars.
With the influx of cast and crew, the existing population of Castel del Monte - 129 people - was more than doubled. When a crew member needed emergency dental work, he had to go to a nearby town as the answering machine of Castel del Monte's dentist declared that he was "unavailable, due to his appearance in a Hollywood film."
The film's original title was "Il Americano". Technically, this is not correct Italian - it should be "L'americano" - but Anton Corbijn wanted to use the incorrect title as a homage back to the spaghetti Westerns. Sergio Leone's Once Upon a Time in the West (1968) is seen playing on a TV at one point in the movie.
As Jack sits in the café drinking coffee alone, we hear a song played over the radio. This song is "Tu vuò fà l'americano" ("You pretend to be American"), an Italian song about an Italian man who adopts American affectations.
A lot of the locations the production team had originally hoped to use were ruined in an earthquake suffered in the region. The pace of reconstruction proved too slow so new locations had to be sought out.
George Clooney played a trick on the crew. When the train pulls into Sulmona Introdacqua station, it was set up so the doors would open and the woman step straight into shot. The train driver was given a marker to stop at. Clooney told the driver there was a change of plan and he had to stop 5 m past his marker. Laughter from Clooney and consternation from the crew.
The last time Jack meets Mathilde they get interrupted by the countryside equivalent for a streetcar, which running kids get off. It has "desiderio" painted on the left side. In Italian "desiderio" means "desire".
The beginning of the film was shot in the north of Sweden. The Swedish county mentioned in the film is called Dalarna which is located in the middle of Sweden. However no scene was shot in that county. The county used for shooting is called Jemtland and is located in the north of Sweden. The newspaper Jack gets in his hand at a café in Italy is a Swedish newspaper.
The small red truck-like vehicle that 'Jack' repairs for 'Father Benedetto' is a vintage Moto Guzzi Ercole. It is essentially a three-wheeled motorcycle with a small pickup bed. This particular Ercole (Italian for "Hercules") is equipped with the optional 2-door cab.
The James Bond movie Spectre (2015) features a story element referring to "L'Americain" [The American]. Reportedly, star George Clooney was once considered for the role of James Bond for Casino Royale (2006). Apparently, producer Barbara Broccoli originally wanted Clooney for the role and eventually met him to discuss the part, but Clooney turned it down, saying something like I'm American and it wasn't right for James Bond, James Bond is English and not American. Clooney has been likened to the late great American movie star Cary Grant who was considered to play James Bond during the 1960s and was the best man at the wedding of Dana Broccoli and co-founding Bond film franchise producer Albert R. Broccoli, and father of Barbara. Clooney later starred in this espionage picture called The American (2010) [ "L'Americain"] which was released between Quantum of Solace (2008) and Skyfall (2012) in a year where there would have been a Bond movie with a two year cycle.