From 1937 to 1951 he collaborated on a series of films with Veit Harlan.
Frustrated at the delay, Goebbels removed him in favor of Veit Harlan.
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In 1943 Caspar was engaged by the director Veit Harlan to play the young August Neidhardt von Gneisenau, who in 1807 defended the Prussian fortress town of Kolberg against the French during the Napoleonic Wars, in Kolberg, an epic film produced on the orders of Goebbels.