The original idea and inspiration for the film stemmed from a Japanese short story concerning two characters who often walk by each other in a stairwell, but do not converse. In that story the characters end up committing suicide.
During filming, Kar-Wai Wong improvised often with the actors, crafting the story and mood of the film as he went along. Originally, "In the Mood for Love" was a much more obvious romance film, with the actors throwing witty dialog at each other and engaging in several scenes of love-making. Eventually, the actors and director decided to tone the mood down to the more subtle version that was released in theaters.
Director Kar-Wai Wong found the English title for "In the Mood for Love" while listening to a song from a Bryan Ferry CD with a similar title, "I'm in the Mood for Love". It is a cover of a 1930s song with the same title, Kar-Wai Wong used the title and the song in an early Hong Kong trailer of the film, and it was also used in the USA trailer of the film.
Despite being the two primary antagonists of the piece, the characters of Mr. Chan and Mrs. Chow are never physically seen on-screen. Roy Cheung and Paulyn Sun, who play these respective characters, actually filmed scenes as their characters which were edited out of the final cut (these scenes are present on the DVD and Blu-ray as deleted scenes) and their presence only remains in the film as voice cameos.