Even though the screenplay may be the “most vital” element in a film, as Hitchcock clearly states (see How to Watch a Movie: Part One), there are many more elements to study when watching a film:
(2) Acting. Good acting – you know it when you see it. Great actors, however, can do very little to improve bad material, but mediocre actors can shine quite bright when working with awesome material. So if a film has a fantastic screenplay with amazing talent – even if the execution of the many other elements are less than perfect – the movie should still be a success.
(3) Directing. Great directors are not necessarily “control freaks.” But control is an essential part of their job description. Directing is all about vision. And great directors are master mechanics of (a) controlling a crystal clear vision (b) surrounding themselves with people who can execute that vision, and (c) making sure that all individuals involved understand and embrace that vision. If a movie loses its vision, or even worse, never had one to begin with… Blame the director!