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.
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.