Natalie Portman was originally turned down by Todd M. Thaler (the casting director) due to being too young, but she returned to the auditions and performed the scene where Mathilda laments the loss of her brother. Luc Besson was so impressed with the depth of emotion she summoned during the audition that he gave her the role.
During the filming involving all of the police cars on the street, a man ran from a store he had just robbed. When he encountered the movie set by accident, he saw all of the "police" and gave himself up to a bunch of uniformed extras.
Mathilda checks herself and Léon in under the name "MacGuffin". "MacGuffin" is a movie term coined by Alfred Hitchcock for a trivial element in a movie which serves no other purpose than to drive the plot forwards.
The original script had more scenes with "awkward sexual tension" between Mathilda and Léon. These scenes were later cut out for the American release dubbed "The Professional", but were included in the 1996 European release, as well as in the deleted scenes of the special edition DVD. They were reintegrated back into the film for the 'International Cut', which is now available on DVD.
The pistols that Léon use in the film are Beretta 92FS's with AL-GI-MEC compensators added on. In some scenes, he affixes them with sound suppressor, with threads that extend past the compensator cuts to cancel out the escaping gases that could potentially escape and produce a loud gunshot. In the final firefight, one of Léon's Berettas is seen with a stainless or "Inox" frame.
According to Jean Reno, he decided to play Léon as if he were a little mentally slow and emotionally repressed. He felt that this would make audiences relax and realize that he wasn't someone who would take advantage of a vulnerable young girl. Reno claims that for Léon, the possibility of a physical relationship with Mathilda is not even conceivable, and as such, during the scenes when such a relationship is discussed, Reno very much allowed Mathilda to be emotionally in control of the scenes.
When the villains are turning Mathilda's home upside down, the crook with the dreadlocks finds a reggae album in the record collection. The record he is holding up and referring to as "cool" is "Marcus Children" by Winston Rodney (aka Burning Spear) from 1978.
Natalie Portman's parents were extremely worried about the smoking scenes in the film, and before they allowed Natalie agree 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, Portman 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.').
Léon Gaumont was a French inventor developing cinematographic techniques in the late 19th century alongside with the Lumière brothers. Gaumont is the name of the production company that made this film.
According to Luc Besson, the role of Léon was always intended for Jean Reno and no one else. However, according to the Fact Track on the Deluxe Edition DVD, both Mel Gibson and Keanu Reeves were extremely interested in the role.
Natalie Portman stated that the scene in which she dressed up as Marilyn Monroe was inspired by a scene she saw in Wayne's World. She admitted that at the time Léon was filmed she actually had never watched any movie starring Monroe.
According to Patrice Ledoux, Luc Besson planned Léon as filler. At the time, he had already started working on The Fifth Element, but production was delayed due to Bruce Willis's 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. Ironically, Léon is now generally considered to be a far superior film to The Fifth Element.
The second shot (a tracking shot traveling down a New York street without stopping) could only be accomplished after carefully studying the pattern of the traffic signals so as to insure the camera truck didn't encounter any red lights.
During the scene when Stansfield (Gary Oldman) 'interrogates' Mathilda's father (Michael Badalucco), Oldman smells the father, and gets extremely physically close to him. According to Badalucco, he had no idea that Oldman was going to smell him, nor that he was going to get as close as he did. Badalucco says that in the film, his look of discomfort during the scene is completely genuine, as he felt decidedly intimidated by Oldman, and the physical proximity between the two made him very nervous.
According to actress Maïwenn, part of the film is based on her romantic relationship with director Luc Besson. Le Besco (who plays the blond prostitute in the opening scene) was engaged to writer/director at the time the film was made. Le Besco had met Besson when she was 11, and had fallen in love with him when she was 15 (Besson was 32 at the time).
When the film was first tested in LA, the version that was screened incuded a short scene where Mathilda asks Léon to be her lover. However, the audience became extremely uncomfortable and began to laugh nervously, completely destroying the tone of the film. The film received terrible test scores at the screening, and as such, producer Patrice Ledoux and writer/director Luc Besson decided to cut the scene for theatrical release.
Luc Besson got the idea of doing this movie while working on his previous movie, La Femme Nikita. In that film's third act, Victor the Cleaner (Jean Reno) appears to deal with the aftermath of Nikita's botched mission. Realizing the potential of the character was underused in that movie, Besson decided to create a story that focused on the activities of such a character. Both Victor and Leon appear dressed in a long wool coat, sunglasses and a knit cap. The film's working title was "The Cleaner".
French actor Samy Naceri has in this film one of his first roles. He plays a bit part, a SWAT, and was only ten days on the set. As Luc Besson wrote the screenplay for the film Taxi, he always wanted Samy for the lead role, because he was much impressed by the short cooperation with him.
Keith A. Glascoe, who played the enormous Benny, or 3rd Stansfield Man, later became a member of the New York Fire Department, Ladder Company 21 in Hells Kitchen. Courageously he died in the collapse of the World Trade Center towers on Sept. 11, 2001.
The scene in which Gary Oldman's character 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 trivia item below may give away important plot points.
The original script had a much darker ending. After Stansfield shoots Leon, Mathilda performs the grenade ring trick (seen in one of the restored scenes of "Version Integrale") and opens her jacket to reveal the live grenades. It was changed by Luc Besson, fearing that the audience would not accept Mathilda's transformation from innocent girl to crazed juvenile killer. Besson had never intended such a transformation for the character.