The auditorium and stage of the Paris Opera House seen here was the same set built for the 1925 version. It still stands at Universal Studios today, and has been used for countless other productions, including Thoroughly Modern Millie (1967) and The Sting (1973). It is the oldest remaining film set in the world.
The original script revealed Claudin to be Christine's father, who abandoned her and her mother in order to pursue a musical career. When this was excised from the final film, it left Claudin's obsession with Christine unexplained.
On 21 May 1943, the finished film was rejected by the Hays Office because of a "number of unacceptable breast shots of Christine" in her dressing room. It has not been determined if the offending scenes were deleted or re-shot, but the film was released in Aug 1943 with Production Code Administration approval.
In the scene where the three heroes escape the Phantom's crumbling lair (which involve the three characters running from a cave in) only Susanna Foster actually appeared in the scene, the two male stars were deemed too important to film such a risky scene and had stunt doubles.
Because the war in Europe made it so difficult to track down who had the rights to most operas (coupled with the studio's reluctance to pay the required royalties), all the operas performed in the film were either in the public domain (i.e., copyrights had expired and anyone could use them without paying royalties)) or were based on classical music that was in the public domain. The filmmakers were able to slip in a reference to the opera "Faust" (which featured heavily in the original novel) by having Christine appear in the Marguerite costume as she comes off stage at the end of the film.
Of the three "operas" in the film, only the first, "Marta," by Friedrich von Flotow, is an actual opera. The second, "Amore et Gloire," is adapted from music originally written for piano by Frédéric Chopin: the overture and opening chorus is taken from the "Military" Polonaise in A major, Op. 40, No. 1; the duet between Anatole and Biancarolli is taken from the Waltz in C-sharp minor, Op. 64, No. 2; and the music for Christine's aria/duet with Anatole is taken from the Nocturne in E-flat major, Op. 9, No. 2. The music of the third opera, "Le Prince Masque de Caucasie," is actually excerpts from the finale of Pyotr Ilyich Tchaikovsky's "Fourth Symphony".
Despite his fearsome reputation Susanna Foster remembered 'Jack Pierce' as being a sweet caring man that took the time to teach her how to apply her makeup to best suit her features, despite the fact that he was not assigned to work with her. Any time that he saw her where she was not made up as he had suggested he would pretend to be mad and tell her off, saying "You are not doing what I said".