I've just re-watched this movie, and listened to the highly enlightening audio commentary by Alan Pakula and Robert Mulligan (the producer and director).
First off, I'm afraid I have to be the crotchety curmudgeon because the one thing that pops into my head is: they don't make them like this any more.
Well, it's true, they don't. So shoot me.
Beyond the obvious fact that it's in black and white: The pacing compared to today's movies is very leisurely, without rapid cutting and with rather sparse camera movements. The story and the mood are given plenty of time to build up. The sountrack is very quiet, compared to today's films. Nowadays filmmakers seem to live in fear that the audience would fall asleep without the constant presence of a crackling, bustling Foley track and thudding music. In TKaM, even Elmer Bernstein's music track is quiet, and all the more powerful for that, proving that even in music, less is more.
There are three violent deaths in the film, but the only one that happens on screen is the shooting of a rabid dog.
In the whole movie, there is only one special effects shot: the one in which Boo carries the injured Jem to the house. The sparkling, moonlit leaves had to be superimposed on him in the printer.
For a Hollywood movie, there are very few closeups, and when they come, they pack a huge whallop. Again, less is more.
Chefs say that the simplest food, with the fewest ingredients, can be the most impressive, but only if you do everything perfectly. These guys did everything perfectly.
This could be one film where you could make a case that the real auteur is not the director but the literal author -- the person who wrote the screenplay. Like Rod Serling, Horton Foote learned his craft in the Golden Age of live television drama (Playhouse 90, etc.). This produced a lot of writing characterized by ideas and serious issues and a zero level of spectacle. A number of the actors were live theater actors brought in by Foote, which is why so many of them are unfamiliar faces (something that adds greatly to the realism of the film, for me). Mercifully, Gregory Peck was the only one in the cast who was a star at the time (it was the film debut of both William Windom and Robert Duval).
One thing I first noticed on this viewing was the highly effective way Peck's summation speech to the jury was presented. Except at the beginning and the end, I'm pretty sure there are no reaction shots of the jurors at all. Instead, we see Peck from the jury's point of view. He leans toward us, almost as if he were speaking to the movie audience itself. We have the impression that we hold the life of the accused man in our own hands, and through this issue must wrestle with the injustice of the world. There are a number of factors that make this one of the greatest single speeches in any film. The understated-but-brilliant camera work and editing are among the less obvious ones.
Like I say, less is more.