In the movies, [voice] projection isn't usually required. (Pay no attention to the sound technician; he always has a problem.) Sensitive microphones can pick up the softest delivery...[A]s the occasion calls for it, you can speak in a barely audible whisper or you can let it rip and be as loud as you want. The sound technician, however, has to know in advance because he's wearing the earphones. I remember I was doing a little sound feature picture in which I played some sort of hoodlum. I had to come up behind a guy and whisper, "You're gonna die," and then shoot him in the back. The sound technician had his equipment turned up high so he could hear the whisper. I forgot to say the line and fired the shot. The sound technician had full volume on his earphones, and the sound of the shot nearly burst his eardrums. He went away and took a lot of aspirin.

Acting in Film: An Actor's Take on Movie Making (Revised Expanded Edition) by Michael Caine (1997)