The Danza: Reggae singer Willi One Blood plays one of Stansfield's goons; another character calls him "Willi", and Stansfield calls him "Blood".
Dawson Casting: Luc Besson averted this with Natalie Portman (who was eleven years old during filming); he resisted early attempts to cast older actresses despite knowing such a call would have made things easier for him.
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).
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. 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.
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 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 a 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).
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.
In the original script, Leon accidentally walks in on Mathilda when she's in the shower.
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.').
Mathilda wants to kill Stansfield in order to avenge the death of her brother, who was the only member of her family that she cared about. When you think about it, this means her revenge was partially directed at the wrong person. That's because although Stansfield kills the rest of Mathilda's family (and is the leader of the gang), her brother is actually killed by Stansfield's henchman Willi Blood (you can hear Malky scolding him for it when Mathilda walks by the entrance to the apartment, right after the raid). This means that she is actually present to witness the death of her brother's killer, as she is in the room when Léon shoots him (although it should be noted that even if Stansfield lived to face trial, he would probably get charged with murder in the death of the brother, even if he wasn't the one who pulled the trigger in that one particular case).