By components, I was referring to directing, writing, and acting that were being discussed. For me I often see a movie I really like but can't point out the role of each of those things in making it a great movie.
By gains I was talking about technical advances that make filming easier. Example, I read that the first star wars movies, the light from the blaster took a lot of time to create and film but the newer films computers do this in seconds.
In films, the opportunity for actors to improvise--should they even want to--is going to follow from the director. For instance, Billy Wilder and his writing partners
never allowed for ad-libbing. Actors read every fucking word in the script. You can make sense of this (perhaps draconian) insistence on the scriptedness of the film in a few ways. First, Wilder got his start as a Hollywood writer in the 1930s, an era in which writers were agitating for guild recognition, reformed procedures for screen credits, and some measure of creative control vis-a-vis directors and producers. Wilder itched to become a director because he was pissed that another director butchered his dialogue. And so he never gave up that stranglehold over production. At the same time, though, a strict adherence to the script was consistent with studio imperatives: what the script provides is an efficient blueprint for budgeting a film and filming it as quickly and as well as possible.
If you really want to know how Old Hollywood directors exerted control over filmmaking, it was through principal photography. If you want to thumb your nose at a producer (who, in those days, had final say over the cut), then you don't shoot coverage of scenes (i.e., you don't shoot the entire scene in a "master" long shot followed by individual medium close-up and/or close-up dialogue/reaction shots). Coverage is a hedge against realizing that you don't have the right material in the editing room. Instead, you "shoot it in the can," which is to say, you just shoot the exact shots that you want in the final cut. In short, you do not give the producer the extra material to significantly change how the film looks or how the story is told. What's notable about this argument is that it does not rehearse old cliches about the long take as an exercise of authorial intentionality. If anything, the long take reflected the myriad skills that the old studios could coordinate and make cooperate--the studios could marshal enormous technical skill to pull off the complexities of those shots in the days before digital editing made every show think another fucking one-er was the aesthetic be-all and end-all.
Put another way, Old Hollywood directors exerted control by limiting the producers' options. Conversely, contemporary directors often presume to exercise control by expanding the footage options available to themselves in the editing room.
Flash forward to the 1970s, when Wilder is still (somehow) working, and you can find plenty of counterexamples of directors who now encourage improvisation. In the main, I'd chalk this change up to the development of naturalist acting techniques in the 1950s (i.e. the Method, Strasberg, Meisner, Adler), which often emphasized the actor's "interpretation" of a role as integral to successful storytelling. And so directors like Altman regard spontaneity--in acting and in camerawork (where spontaneity basically means a set of specific techniques)--as the sine qua non of dramatic truthfulness. These naturalist premises are everywhere--Lumet wants improvisation in rehearsal to lock down a script (note: it's a cheaper way to work because it makes for faster shooting with full crew; Mike Leigh works the same way nowadays); Pollack wants unrehearsed actors to 'naturally' act in just a few takes (and he's got bigger budgets than Lumet because bigger stars); Cassavetes and May go full improv (no one is making any money). Notably, improvisation is another front in any undeclared war between above-the-line film workers. William Goldman calls Pollack a "writer killer" for how the director re-interprets scripts. In turn, improvisation means that actors are not the puppets of directors and screenwriters, but the originators of something artful themselves.
(As an aside, to ding
The Wire for its non-professional actors is probably to misunderstand Simon's aesthetic aims and, moreover, the sense in which he imagines himself working through neo-realist premises).
Rutgers University Press has a good edited series on some of this stuff--there are individual books on directing, producing, cinematography, acting, screenwriting, editing, and art direction. Each one moves chronologically through the history of Hollywood filmmaking to clarify how the organization of the wider film industry bears on the work of these various professions. I'd say that the directing, cinematography, and acting books are the most informative bang for your buck.