Henry Daniell couldn't fence. The climactic duel had to be filmed using a double and skillful inter-cutting.
The scenes of Doña Maria's (Brenda Marshall's) carriage travelling through the countryside were taken from another film. The movie had to be darkened to disguise the fact that the carriage depicted was clearly too modern for this movie's Elizabethan setting.
Basil Rathbone, who had previously fought Errol Flynn in Capitán Sangre (1935) and Las aventuras de Robin Hood (1938), refused the role of Lord Wolfingham.
The Panamanian sequences were tinted in sepia, as was done with the Kansas scenes in MGM's El mago de Oz (1939). Television prints of this movie were entirely in black and white. The sepia was intended to suggest the sweltering heat of the jungles in Panama.
The beautifully crafted costumes were made for the Errol Flynn movie, La vida privada de Elizabeth y Essex (1939). Re-using them saved Warner Brothers a huge amount of money, since the costumes were heavily researched, meticulously created, and very expensive.