This film as well as Sundays and Cybele (1962), both released in late 1962, are the first to feature the now famous "Adagio in G minor" by Tomaso Albinoni (1671-1751) which was first published in 1957.
The "pin-screen," also called the "pin-board," used in the opening and closing sequences was invented by Alexander Alexeieff in the early 1930's. It is a board with pins stuck in it at regular intervals. The pins can be raised or lowered to form an image, which can then be lit and photographed. By manipulating the pins and photographing them one frame at a time, the device can be used for animation, and though it was not so used in "The Trial" Alexeieff and Claire Parker made at least two short animated films using the pin-screen, Une nuit sur le mont chauve (1933) and Le nez (1963).
Katina Paxinou was cast in a small role in the film, as a computer analyst. Her part took up only one scene and was cut out, but Orson Welles claimed that he had left her name in the advertising for the film so that her admirers might be persuaded to visit the film a second time to see if they had somehow missed her the first time round. (Her scene is included in the published version of his script).
The trivia item below may give away important plot points.
In the novel, K is killed with the knife, putting up little fight, but Orson Welles felt that was inappropriate with the Holocaust in recent memory - particularly given the frequent interpretation that K is a Jew, so he changed it for the film.