In the beginning of the film, Lisa is 16 and played by Joan Fontaine who was 30 years old at the time the film was made.
In the film, Lisa is at least several years younger than Stefan Brand. In reality, Joan Fontaine is older than Louis Jourdan.
The song Brand is playing while Lisa is listening to him in her room is Liszt's Un sospiro.
The opera Lisa and Johann go to is "The Magic Flute" by Mozart.