The Prisoner of Zenda (1937)

This is a classic swashbuckler. Rudolph Rassendyll, Rudolf V's identical distant cousin, is asked to risk his life and impersonate the would-be king when his relative is kidnapped before his impending coronation. If Rudolf V isn't present at the ceremony, he will forfeit the crown to his younger brother. Complications ensue when Princess Flavia, the cousin's betrothed, begins to notice a "personality change" in her fiancé.