He is probably best known in popular culture, particularly to English-speaking audiences, for playing the archetypal mad scientist role of C. A. Rotwang in Lang's Metropolis and as the criminal genius Doctor Mabuse.

[1] His father, Hermann Rudolf Klein, was a military lawyer who served as auditeur (the equivalent of Judge Advocate General) for the 15th Division of the Prussian Army, garrisoned at Cologne.

He also began taking acting lessons from Hans Siebert, a veteran of Vienna's Burgtheater, and made his stage debut in 1909, playing Cassius in Julius Caesar in Halberstadt.

The confusion may have stemmed from Klein-Rogge's skill in disguising his appearance with theatrical makeup, wigs and prosthetics, which has led at least one movie historian to dub him Germany's Lon Chaney.

[2] Despite the split, Klein-Rogge made several films that were written by von Harbou and directed by Lang, including Destiny, Dr. Mabuse the Gambler, Die Nibelungen, Metropolis and Spies.

[1] Klein-Rogge played the lead roles in two films written and directed by von Harbou: Elisabeth und der Narr and Hanneles Himmelfahrt.