I am grateful to Peter for the following piece of information:
As you know, Cecil B de Mille was a great producer of biblical epics in the Hollywood style. It seems that, when filming the Ten Commandments, he spared no expense on the scene where the Israelites cross the Red Sea. He created a gigantic film set, hired literally thousands of extras, had the special effects and the sound recorder boys at the ready, and so on. When he deemed it ready for the shoot, he was about to call "Action!" when the heavens opened and God's voice boomed down from the clouds: "Ready when you are, Mr de Mille."