Fritz Lang consciously choose to introduce Willy Fritsch as a scruffy homeless person, for Fritsch was most known from his previous films as the elegant gentleman he later becomes in this movie as well.
Gerda Maurus, whose film-debut "Spione" was, and who met Fritz Lang for the first time here, later had a long relationship with the director, eventually causing his divorce from Thea von Harbou, who at the time was his wife and remained his regular co-author up until The Testament of Dr. Mabuse, Lang's last German film before emigration to the U.S..
UFA insisted on the film being very cheap in the making as Fritz Lang's previous film Metropolis had brought the studio to near bankruptcy. Lang therefore choose to do most of the shots in narrow settings with lots of close ups, as no big sets had to be built up for that way of filming. Fortunately "Spione" became a huge success.
When Agent 326 trails Sonya to Col. Jellusic's apartment you can see posters for Fritz Lang's prior film, Metropolis, on the street walls in the background.
Fritz Lang:
[Regulars]
Actors Rudolf Klein-Rogge and Fritz Rasp in leading roles, Thea von Harbou co-writing the script.