He devoted himself to professional acting ever since, appearing on the stages of theaters in Flensburg, Hanau, Baden-Baden, Würzburg and Eger.

[1] While performing in the Eisenach Theater, he met his future wife, actress Martha Beschort Archived 2011-07-17 at the Wayback Machine.

Diez, who originally was a member of the Guild of the German Stage, joined the Profintern-oriented Revolutionary Trade Union Opposition and was elected as chairman of the theater's branch.

[2] The St. Gallen Theater's manager, Theo Modes, was a supporter of the Third Reich, and Diez was "completely isolated" in his place of work.

[5] By the end of the war, Diez headed the St. Gallen fraction of the Democratic Union of Germans in Switzerland, an offshoot of the Committee.

"[1] He played the character on screen and in television in ten different productions - among them Ernst Thälmann - Führer seiner Klasse (1955), I, Justice, Frozen Flashes (both 1967), Liberation (1970-1), Seventeen Moments of Spring (1973), Take Aim (1974) and Soldiers of Freedom (1977).

[11] Author Charles P. Mitchell wrote that the actor was "Eastern Europe's equivalent to Bobby Watson in terms of the frequency of his Hitler appearances.