[2] He subsequently travelled to Berlin and took acting lessons from stage actor Paul Biensfeldt and then relocated briefly to Hamburg where he performed at the Harburger Theater.

[2] Kastner cemented his romantic image by appearing as the ardent suitor to popular actress Dorrit Weixler in a number of films of the era.

Male filmgoers were less fond of Kastner's image of a handsome dandy and gave him the derisive nickname "Kleiderbügel" ("coat hanger") – a reference to his slim build and fashionable wardrobe.

In 1921, Kastner was voted "The Best German Actor" in a magazine poll[3] and he began appearing that year in a film serial called Der Silberkönig (The Silver King) opposite leading lady Ossi Oswalda.

No longer able to play the young, seductive bon vivant type which had made him famous, his roles in films grew smaller, although he still had a measure of success in such films as Karl Grune's 1926 drama Die Brüder Schellenberg with Conrad Veidt, Lil Dagover and Liane Haid and the Jacob Fleck and Luise Fleck-directed Der Orlow with Iván Petrovich and Hans Junkermann in 1927.

After failing to garner any more film roles because of his speech impediment, he tried to revive his career by touring German theatres and permitting female members of the audience to get onstage and have their photograph taken with their past idol.