When Carla meets Guido at the train station, she says that she stayed at hotel with her parents during the war, and there were a lot of Germans there. This references the song "The Germans at the Spa" which was in the play, but not used in the film.
When Judi Dench was cast, her manager (who also manages Daniel Day-Lewis) sent the script to him, and he in turn called up the director Rob Marshall. Marshall stated in an interview that he thinks Day Lewis is the "best" actor around, and that he never considered offering him the part because he didn't think he would take it. Day Lewis himself was keen to take a break after his last film, but said he knew that making the film would be an unbelievable experience.
The music in Nine is a mixture of modern musical theatre and italian operatic styles. The music uses this mixture to help evoke the era and setting of the film.
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A Note Regarding Spoilers
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The following FAQ entries may contain spoilers. Only the biggest ones (if any) will be covered with spoiler tags. Spoiler tags have been used sparingly in order to make the page more readable.