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.
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 code that Leon gives Matilda to knock on the door when she returns from getting more milk is two knocks, then one, then two knocks again. This is the telephone area code for Manhattan, which is where the story takes place.
Immediately prior to Mathilda's capture by the SWAT team, she is holding the keys in her left hand. Even though she had no opportunity to change hands, they are in her right hand when she hands them to the SWAT team.
The bathroom mirror disappears and reappears during Mathilda's charades game. It is on the wall for 'Madonna', off the wall for Marilyn Monroe, back on the wall for Charles Chaplin, and off again for 'Gene Kelly'.
As part of his code of conduct, Leon's main rule is "no women, no kids," and Mathilda fits both categories. He knew that she would suffer the same fate as her family at the hands of Stansfield and his men if he didn't open the door. Although he would not be the one who pulled the trigger, he would have felt responsible for her death, since he could have prevented it.
What guns are used in the film?
See here for a detailed overview of all of the weapons seen in the film.
What are the differences between the theatrical cut, the director's cut, and the extended version?