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Some Academy Award categories are pretty easy to figure out, such as Best Actor/Actress and Costume Design. However, one that has always puzzled me is Best Director. While we know that a director is generally considered the author of a film, their exact role is not always clear, involving and affecting elements that are already awarded in separate categories (including writing, acting, cinematography, and sound design).
While the director's contribution might be clear to anybody working on the movie in question, it becomes a completely different matter when outside observers, like the Oscar jury, need to evaluate it. It seems impossible for them to decide, based purely on the finished work, how much was actually down to the director.
Given that much or all of a director's role cannot be clearly separated from those of other crew members and unambiguously observed in the finished work, how does the Oscar jury decide which movies should receive an Oscar for direction?
In other words, how does the jury know that the result is not simply down to the writing, the acting, the cinematography, or anything else that the director cannot necessarily be credited with? Is there some unique task performed only by the director that the jury can also observe? Is it more of an overall assessment of a movie's quality that can also be clearly separated from the more general Best Picture award? Do they consult other sources than just the finished work?
Note that this is not a question about what a director actually does but rather how the Oscar jury, as outside observers, can disentangle the director's role from those of other crew members with overlapping responsibilities.