Charlton Heston spent up to five hours a day at rehearsals conducting Ludwig van Beethoven's Symphony No. 5 in C-, Op. 67, Allegro con brio, the music performed over the opening credits. In his diaries, Heston describes it as better than parting the Red Sea.
Charlton Heston wanted Anne Heywood or Jessica Walter for the role of Annabelle Rice, ultimately portrayed by Kathryn Hays.
Anton Diffring would be cast once again as a Nazi officer in the War classic, Where Eagles Dare (1968), the following year.
When Evans tells the orchestra, "Let's try the Tchaikovsky," what they play as Sgt. Calloway climbs up the chapel belfry is indeed a piece by Pyotr Ilyich Tchaikovsky: the Overture to the ballet, Swan Lake. The mentioned Brahms is also indeed a Johannes Brahms piece they play when Annabelle has dinner with Schiller: the fourth movement of his Symphony No. 1 in C-, Op. 68. All of the musical references made in the movie are accurate, such as when Evans tells the orchestra, "Once again please from bar 19..." of the Tchaikovsky, they do indeed start at bar 19.