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 met Michelle Pfeiffer years before, in the early 80s he had gone on a few dates with Michelle's sister, actress Dedee Pfeiffer. He and DeeDee 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.
Tom Cruise was originally eyed to take the male lead role. George Clooney stepped in when he turned it down.