Charles Laughton, who could be moody and difficult, was apparently a dream to work with, throwing himself into the role with dedication and delight. Billy Wilder later recalled a day that was set aside just for shooting reaction shots of the jury and courtroom crowd (composed of extras hired only for the day). Normally, the assistant director would read the actors' lines, and the extras would react. However, Laughton, who was fascinated with the whole process of filmmaking, begged to help. So he came in on his day off and read all of the off-camera speeches to the jury members. He not only read his part but also the judge's, the prosecutor's, and even Marlene Dietrich's. According to biographer Maurice Zolotow in his book "Billy Wilder in Hollywood", "it was an exhibition of craftsmanship such as Wilder had never seen. He believes that Charles Laughton had the greatest technical range and power of any actor, man or woman, whom he has known."
At previews, audience members received, and were asked to sign, cards that read, "I solemnly swear I will not reveal the ending of Witness for the Prosecution."
Sir Alfred Hitchcock said "Many times, people have told me how much they enjoyed Witness for the Prosecution. They thought it was my film instead of Billy Wilder's. And Wilder told me people asked him about Der Fall Paradin (1947), thinking he had done it."
Marlene Dietrich was so certain she would be nominated for an Academy Award for her performance as Christine Vole that she recorded a new introduction to her Las Vegas show mentioning her nomination. She was crushed after she was not nominated.
The press book, reviews, and various articles about the production stated that the principal cast members themselves did not even know the ending of the film until the last day of shooting, when the final 10 pages of the script were presented to them.