It's remarkable how Peter Lorre could convey such different emotions through his expressions; he managed a terrifying portrayal of madness and decay in M, and then, just a decade later using essentially the same features, presented an innocent and open smile as Janos Szabo, the Hungarian immigrant in The Face Behind the Mask. Janos, who considers himself a “mechanical genius” and is also a watchmaker, arrives in New York hoping to achieve the American dream. Surprisingly, the city welcomes him reasonably well; he quickly finds employment at a hotel that provides both work and lodging. However, disaster strikes when the hotel burns down, leaving Janos with severe facial disfigurement. The same city that initially showed him kindness now shuns him because of his appearance. Unable to find legitimate work, Janos is compelled to use his manual skills for criminal activities. Even a great deal of money, whether earned honestly or not, can't repair the damage to his face; Janos, now calling himself Johnny, begins wearing what’s best described as a mask resembling Peter Lorre (though it isn't Peter Lorre pretending to wear one). Johnny eventually finds love with a blind woman named Helen Williams (played by Evelyn Keyes), demonstrating that even a good woman’s affection can redeem someone. He abandons his life of crime, much to the disappointment of his former partners, who eliminate Helen in an early example of the tropes involving disposable women and external threats. The story concludes with Johnny flying his ex-associates out to the desert and leaving them there—including himself—to face a slow demise. Lorre’s performance might have made this lesser noir film worthwhile on its own, if not for the fact that his incredibly expressive features are rendered unable to convey emotion (it's similar to how modern digital technology is used to de-age actors). Often we hear Lorre’s voice instead, which could have been effective—he possessed a uniquely recognizable voice as well as his face, like Claude Rains did in The Invisible Man, although in that film it was understood from the start that he wouldn't be seen much.

Source on TMDB