Director Fritz Lang (1890-1976) was a major player in the film aspect of Germany's school of expressionism, which came over to the USA and developed into film noir. I read that propaganda minister Joseph Goebbels first offered Lang the position eventually filled by notorious Leni Riefenstahl, in the Third Reich.
Lang was a soldier who suffered from shell shock (PTSD), which may have affected his brutal directing style that made some actors/actresses refuse to work with him. His monocle was an affectation, promoting his tough persona.
The Encyclopedia Britannica describes his films as "masterpieces of visual composition and expressionistic suspense...noirish masterpieces of menace—tone poems of fear and fate that have stood the test of time."
If you appreciate film noir and have not yet seen this one, enjoy!!! (Edward G. Robinson, Joan Bennett, Dan Duryea.)