Auteurs are as diverse as humans, and the tools appropriate for analysing Ford are not those for Rossellini. A theory valid for all auteurs is as useless as one for all cinema - whose definition changes with every good movie, if it is art, if it is human. If one starts out from theoretical positions, all hope at arriving at a viable experience of auteur in the context of Hollywood cinema 1930-60 is hopeless, like setting out to meet music determined to ignore rhythm. For it is the distinction of an auteur that no suture is required between the regarded and the regarder, anymore than between my fingers and my arm. But there are as many definitions of auteurism as there are auteurs.
People are incorrect to compare a director to an author.
The utility for regarding a director as auteur is the richness of experience that may result.