Friedrich Rudolf Klein, better known as Rudolf Klein-Rogge, was a German film actor, best known for playing sinister figures in films in the 1920s and 1930s as well as being a mainstay in director Fritz Lang's Weimar-era films. 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. Klein-Rogge also appeared in several important French films in the late 1920s and early 1930s.

The mad scientist is a stock character of a scientist who is perceived as "mad, bad and dangerous to know" or "insane" owing to a combination of unusual or unsettling personality traits and the unabashedly ambitious, taboo or hubristic nature of their experiments. As a motif in fiction, the mad scientist may be villainous or antagonistic, benign, or neutral; may be insane, eccentric, or clumsy; and often works with fictional technology or fails to recognise or value common human objections to attempting to play God. Some may have benevolent intentions, even if their actions are dangerous or questionable, which can make them accidental antagonists.