Michael Caine put out an instructional video on movie acting. At one point in it, we see an actor performing a scene from a play. Maybe it was from Alfie. I don't know. He's sitting in an apartment with his girlfriend. His girlfriend leaves the room and goes into the kitchen, and the actor turns and begins speaking to the camera.
Michael Caine stops him. The actor was a stage actor, so when he starts talking to the audience he speaks loudly so they can hear. On film, Caine tells him, you speak quietly, confidentially, so the girlfriend in the kitchen can't hear.
But there are a lot of movies based on plays where they don't do this. I'm talking about old movies. And it's very strange because the movies were generally made with actors and directors who had no connection to the stage production. The actors bellow out their lines as if they were on stage even when it's wildly inappropriate in a movie. Maybe they were trying to recreate the experience of viewing a play.
I watched a family drama called The Dark at the Top of the Stairs. There are a bunch of relatives visiting a couple whose marriage is on the rocks. This is in the '20s. The wife is thinking of taking the children and leaving. She has talked to her sister, Eve Arden, about it. Arden is afraid her sister will try to move in with her, so she takes her husband aside and tells him what he should say if the woman says anything to him about it. The problem is that she bellows out her lines at the top of her lungs. If it was real, everyone in the house would have heard.
Then there was Every Little Crook and Nanny. About a nanny who kidnaps the son of a Mafia don played by Victor Mature.In one scene, we see the nanny and her husband or boyfriend or someone-----I think it was Lynne Redgrave and Paul Sand. They are in an apartment with the kidnapped child. I don't remember if the child even knew at that point that he was being held for ransom. But the couple is shouting every word they say, even things that are supposed to be spoken normally. The kid can hear, the neighbors can hear, anyone who happens to be walking past down the hallway can hear them shouting as loud as they can about the kidnapping they committed.
There was one scene I remember from The Bad Seed. Leroy, the handyman, is working, looking up and talking loudly to the camera. There was no way to do it realistically in a movie, but they could have made an effort---had him mutter to himself as he worked.