During the shooting, Robert Donat had a severe attack of asthma and the film was delayed for almost a month. The producers wanted to replace him, but Marlene Dietrich refused. According to Robert Osborne, host of Turner Classic Movies, Dietrich waived her salary during Donat's illness and nursed him until he was well enough to return to filming.
During the bathtub scene, Marlene Dietrich slipped on a bar of soap, falling naked and spreadeagled before cast and crew. Ever the professional, she picked herself up, laughed and continued shooting. A day or two later, a newspaper article recounted the incident, with the headline dubbing her "Countess Without Armour."
The poem quoted by Fothergill, aka Ouronov, is "Prospice" by Robert Browning. The one quoted by the Countess is "I have outlasted all desire" (1821) by Alexander Pushkin.
The forest built for the film inside one of Denham's largest sound studios stood there so long that the undergrowth took root.
Marlene Dietrich was imported at great expense to star opposite Robert Donat. Dietrich was eager to work with Donat on Knight Without Armor (1937) because she believed he would be as romantic and irresistible in person as she found him on screen. "He is so beautiful!" Dietrich's daughter Maria later recalled her as saying. Dietrich was disappointed to find that Donat was married, that he wasn't a bon vivant, and that his asthma condition severely hindered his work. Nevertheless, she "rehearsed with Donat to perfect a dialogue technique to get him through the film. He learned to inhale deeply, [to] speak his dialogue in single, controlled exhalations of breath without a sign he was suffering anything more than the occasional pause in which he thought about England.... Donat got through it with her help.