Actor-Inspired Element: Due to the barebones script written on a short amount of time (one month) and the character-driven nature of the work, a lot of the film and its characters came from Jean Reno, Natalie Portman and Gary Oldman, with a good number of lines and elements being ad-libbed.
To another Besson film La Femme Nikita, which also focuses on a young woman training to be an assassin and features a brutally efficient "cleaner" portrayed by Jean Reno. It also helps that the trailer for the movie uses the same song as Nikita and both movies even share the same composer (Eric Serra).
Cut Song: Composer Eric Serra wrote the song "The Experience of Love" as the end credits theme. Instead, the filmmakers decided to use Sting's "Shape of My Heart" and Serra would later re-use his song for GoldenEye. The melody of Serra's song can be heard in the film and on the soundtrack, via the cue "The Game is Over".
The Danza: Reggae singer Willi One Blood plays one of Stansfield's goons; another character calls him "Willi", and Stansfield calls him "Blood".
Darkhorse Casting: While Gary Oldman, Jean Reno, and Danny Aiello were fairly recognizable to moviegoing audiences, Natalie Portman was a relative newcomer when she was cast as Mathilda and her only experience beforehand was serving as an understudy to Laura Bell Bundy in the off-Broadway musical Ruthless!note Bundy's other understudy was none other than Britney Spears. None of the other actors and actresses were particularly well known, though Michael Badalucco (who played Mathilda's father) would carve out a career playing supporting or bit characters in movies and TV shows. Willi One Blood who plays one of Stansfield's goons was a minor reggae singer.
Executive Veto: The American theatrical version of the film cut certain shocking or morally gray scenes of the film, including the most Lolicon-ish aspects of the Léon/Mathilda relationship and scenes of Mathilda accompanying Léon on his assassinations. These cuts change the film's characterizations drastically: in the edited version, Mathilda remains far more pure and ends the film without blood on her hands, allowing her to return to a reasonably normal life, but in the unedited version, she's helped kill over a dozen people (which doesn't fit with her hesitation to kill Stansfield nor her apparent rehabilitation at the end).
Zigzagged with Natalie Portman. Her character is supposed to be Italian-American judging by her surname and some of her family members having New Jersey accents. However, Portman is Israeli and she moved with her parents to the United States when she was young.
The French Maiwenn Le Besco plays an American prostitute who is the girlfriend of the Fatman.
One for the Money; One for the Art: According to Patrice Ledoux, Luc Besson planned the film as filler. At the time, he had already started working on The Fifth Element, but production was delayed due to Bruce Willis' schedule. Rather than dismiss the production team and lose his creative momentum, Besson wrote Léon. It took him only 30 days to write the script, and the shoot lasted only 90 days. As it turned out, Léon is now generally considered by most viewers to be a far superior film to The Fifth Element.
Romance on the Set: Maïwenn had a small role in the film. She and director Luc Besson had an affair on the set (she was 15, he was 32), which led to marriage. They divorced in 1997 when Besson cheated on Maïwenn with Milla Jovovich on the set of The Fifth Element.
Gary Oldman ad-libbed his iconic shouting of "E-VERY-ONE!!" during Stansfield's Villainous Breakdown as a joke. The scene was left in because of how scarily effective it was.
Similarly, at the start of the movie, Oldman improvised his "sniffing out" the lie of Mathilda's father. Michael Badalucco wasn't expecting any of Oldman's actions during the scene, giving him a look of uneasiness that fit the scene perfectly.
The scene in which Stansfield talks about his appreciation of Beethoven to Mathilda's father was completely improvised. The scene was filmed several times, with Oldman giving a different improvised story on each take.
The first draft script was quite similar to the movie but contained a number of notable differences from the final version:
The original ending involved Mathilda doing the grenade ring trick after Stansfield shot Leon. Luc Besson ultimately rejected the ending because he didn't want to shock audiences seeing Mathilda's transformation from an innocent girl to an unrepentant killer.
In the original script, Mathilda (aged 13 or 14) and Léon became lovers. Luc Besson reportedly altered the script to remove this aspect of the story (possibly due to pressure from Natalie Portman's parents).
Leon accidentally walks in on Mathilda when she's in the shower.
Mathilda used a coded bullet to shoot a fat man reading a newspaper, an ugly businessman and a jogger.
Stansfield was an FBI agent.
Leon and Mathilda watch Fred Astaire and Rita Hayworth dancing together in the 1942 film You Were Never Lovelier.
Tony was given a lot more characterization and backstory as an aging mobster with worsening health and lived at an isolated home with an armored car and bodyguards to protect him.
Leon said "shit" at least three times.
Mathilda used a public phone booth to talk with a girl named Jenny about what happened at her apartment.
Stansfield's meeting with Tony originally ended quite differently: After asking him about his protege Leon and Mathilda, he would kill two of his restaurant employees, Emilio and Giancarlo Rinaldi.
Mathilda played baseball with the kid gang.
Leon kills a plainclothes detective interviewing Mathilda at an apartment.
Leon was 17 when he performed his first hit as an assassin.
Liv Tyler was considered for the part of Mathilda but, at age 15, she was deemed too old. Christina Ricci also tried out for it.
A mind-bending example: prior to being cast in this film, Natalie Portman's only acting credit was as an understudy to Laura Bell Bundy in the off-Broadway musical Ruthless! Bundy's other understudy was an 11-year-old aspiring performer named... Britney Jean Spears.
Write What You Know: The relationship between Leon and Mathilda was based on Luc Besson's relationship with actress and director Maïwenn Le Besco, whom he started dating when he was 31 and she was 15.
Other
Stansfield henchman Benny is played by Keith A. Glascoe, who later became a firefighter on Ladder Company 21 in Hell's Kitchen. He was killed on 9/11 in the collapse of the Twin Towers.
Natalie Portman's parents were extremely worried about the smoking scenes in the film, and before they allowed Natalie to appear, they worked out a contract with Luc Besson which had strict mandates as regards the depiction of smoking; there could only be five smoking scenes in the film, Portman would never be seen to inhale or exhale smoke, and Mathilda would give up during the course of the film. If one watches the film closely, one can see that all of these mandates were rigidly adhered to; there are precisely five smoking scenes, Mathilda is never seen inhaling or drawing on a cigarette, nor is she ever seen exhaling smoke, and Mathilda does indeed give up during the course of the film (in the scene outside the Italian restaurant, when Leon asks her to quit smoking, stop cursing, and not hang out with 'that guy. He looks like a weirdo.').
The close-up of a machine gun being loaded before Luc Besson's cameo character shoots is in fact Léon's flashback recalling the sound when loading a gun in the kitchen of Hotel National.
This is the favorite film of the band alt-J, and they have two songs adapting the plot of the movie: Matilda, which centers on León's final moments as he blows up Stansfield and Leon, which centers on León's battle against the SWAT team sent to kill him in the climax, but both deal with León and Mathilda's relationship.
The Transformers clips seen at some points of the film are, at least according to TF Wiki, from the episode War of the Dinobots.
Leon's houseplant is a Chinese Evergreen (Aglaonema). As a tropical plant it is very sensitive to low temperatures and would not survive a winter outside in New York.
as Nikita and both movies even share the same composer (Eric Serra).