I saw this film right after it came out in 1951 when I was 11 years old and before we had TV. We lived in a small rural community and the nearest town, (one main street) had one movie theater. My folks attended a monthly night time meeting and I would be dropped off at the movie theater to go to the movie while they went to their meeting.
The meeting place was about three blocks away from the movie theater and there was only one street light in town so where they parked the car was always in the dark.
This particular night, "The Day the Earth Stood Still" was showing and being a kid
The reason characters whisper is because the other characters in the scene are not supposed to hear what they have to say. It seems many directors, especially in movies today, seem to think that applies to the audience viewing the movie as well,