The movies Sidney Lumet makes now are the best he's ever made. I'll take Before the Devil Knows You're Dead over Dog Day Afternoon and Find Me Guilty over 12 Angry Men any day of the week. Nothing wrong with being an "actor's director" when it produces images like these.
No one else uses the 1.85 frame now like Lumet does. You couldn't cut an inch off of any of the image in Find Me Guilty. A little cropping, and the whole dynamic is lost. It's like removing a letter from a word: a joke would no longer be funny, a line of dialogue would lose its meaning.
Lumet's relationship to the frame (and his relationship is always to the frame and not the image; the image is not what's he's after -- it's just the result of his work with the frame) is like a director's relationship to a stage. It's a way of presenting these people, who in a Lumet film are always costumed actors, and not figures, bodies, ideas, etc. This is mise-en-scene as presentation of evidence. Every object, face, reaction is evident of something. In Lumet's cinema, the director's job is to prove that the script and the actor's performances are true.
The courtroom drama, as a form, is full of interesting possibilities. The action is confined to a single room but also spread across a very large group of people -- judges, bailiffs, lawyers, defendants, prosecutors, jury members, onlookers, stenographers -- each one of whom must speak in turn and has a very specific set of actions.
We should remember that there's a difference between the trial and the court. Directors interested in the mechanics of the court (and in the form of the courtroom movie) are also usually the ones least interested in passing judgment. Lumet, like Preminger, isn't interested in verdicts or victories. As in Anatomy of a Murder, the verdict in Find Me Guilty (though presented as a surprise) is pretty underwhelming. It's the evidence and the testimonies, and how Vin Diesel's characters undermines them, that form the film.