I've since learned that the major reason why the current movie version of Merrily so perfectly captures the look, sound, and feel of a live stage performance is because (confirming the impression I'd gotten in the theatre) it was filmed on the stage of a live production, with at least some of the filming apparently done before a live audience, with the same cast and director as the live performance, and I learned that it was specifically the most recent (and most successful) Broadway run of the show.
Conventional wisdom in bringing stage productions to the screen has always been that if you just film a stage production, it's going to look . . . flat. It's going to combine the worst attributes of both stage and screen. When Jack Warner filmed Edwards & Stone's 1776 (at his own personal expense!), he insisted on bucking that notion, using as much of the Broadway cast as possible (nearly the entire Broadway cast), but he did it on location and in soundstages, still achieving what (up until now) was the closest a movie had ever come to duplicating the live performance experience.
This goes a lot further.