Both Duvall and Nicholson give inconsistent performances, but you can’t blame them as Kubrick excels at getting the acting he wants.  So the question becomes why, and my theory is to amplify the purposeful disorientation in the film.  Yes The Shining encourages analysis and many worthwhile websites (1) (2) do just that. 

  The central story of isolation leading to madness and murder seems to be cancelled out by ghosts facilitating the madness and murders instead, which again invites contemplation beyond the film’s running time.

  Is it frightening?  It’s more eerie and disconcerting, with a few medium-level shock moments of which my favorites include the boy’s tricycle encounter with the twins and the scariest typewriter and carpet patterns in cinema.

        » Brent Chastain, Top3films.com