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 - though some argue that it does fit the end, as she is just an amoral manipulator).
Throw It In: 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. The other actor wasn't expecting any of Oldman's actions during the scene, giving him a look of uneasiness that fit the scene perfectly.
What Could Have Been: 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 Golden Eye. 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 tranformation from an innocent girl to a killer.