Hugh Jackman took etiquette lessons from 19-century etiquette expert Jane Gibson (who trained actors for such projects as Sense and Sensibility). He also studied ballroom dancing and trained to ride a horse for the film.
When Leopold is chasing Stuart down the stairs and out of his house, Stuart seems to jump a few metres in a second. In the Director's Cut version, the two guys actually bump into Kate there.
The violin piece being performed during Kate and Leopold's rooftop dinner is Jascha Heifetz's transcription of Leopold Godowsky's "Alt-Wien", or "Old Vienna". Godowsky subtitled his work, originally written for solo piano, "Whose yesterdays look backward with a smile through tears".
When Meg Ryan knew that Hugh Jackman was going to spend his wedding anniversary alone, she phoned his wife to invite her to dinner. Reportedly, Jackman had to spend the day working.
The trivia items below may give away important plot points.
In the film's original edit, Stuart was Leopold's descendant, which was Stuart's reason for specifically tracking him down. Criticism that this plot line also meant that Kate was Stuart's ancestor lead to those scenes being cut. In the Director's Cut version, however, this plot line is followed.
In an early version of the screenplay, Leopold is accidentally transported through time in an actual time machine developed by a group of scientists that includes Kate. They have a child (of whom Leopold is unaware) and eventually reunite during the Roaring '20s.