When Maggie Taylor forgets the kitten's name, it was not a part of the script. Mae Whitman actually forgot the kitten's name and stayed in character. The director thought that it was so cute that he kept it in the movie.
Near the end when Melanie bumps into the table in her apartment, it was apparently not in the script. Michelle Pfeiffer's clumsiness was therefore left in the movie.
George Clooney had actually met Michelle Pfeiffer years before, in the early 80s he had gone on a few dates with Michelle's sister actress Dedee Pfeiffer. The two had also co-starred in the 1989 cult action film Red Surf.
George Clooney held Mae Whitman in front of his face to hide that his eye was bruised during a basketball game in the scene when Jack shows up at Melanie's apartment.
The film features many uses of the song "One Fine Day" right throughout including composer James Newton Howard's score which serves as the love theme for Michelle Pfeiffer and George Clooney's characters.