In a great interview, Robert Mitchum talked about the difference
between stage acting and movie acting. He said that when you're acting
in a play, everything on the stage is fake except you. In a movie,
everything is real except you. So it's all about the actor's
relationship to the props. Bad actors hold guns like they're light
objects and good actors hold them like they're heavy.

                                                                   — Dave Hickey

An observation worthy of André Bazin.  It occurs to me that actors in the classic Westerns treated horses as if they were real, while actors in modern Westerns treat them as if they were props.