It's a relatively uncomplicated process for a writer or painter to translate an artistic vision into something tangible. Not easy, but uncomplicated. In contrast a film director can, and probably will, face a multitude of challenges turning an idea into images and sound. Before the camera rolls there's financing to arrange, locations to be selected, cast and crew to be hired. Then there's the unforeseen problems that arise during production -- this could be everything from uncooperative weather to the leading lady coming down with pneumonia. Because there are so many moving parts in the process of making a motion picture a movie that successfully reflects the artistic vision of its director is a happy accident. A master filmmaker like John Ford was able to meet the challenges and create an environment in which such "happy accidents" were the norm.
I love this anecdote from the set of The Quiet Man (1952) which shows Ford at the top of his game.
[Assistant cameraman Earnest] Day got a crash course in Ford's immense practical skills one afternoon when they were shooting a brief scene inside Cong's Catholic church. The Technicolor film of that period was slow, and the light levels in the church were low. Ford could have had [cinematographer Winton] Hoch boost the light, but that would have destroyed the reflective, noir mood he wanted. Ford instructed Hoch to lower the camera speed to about twelve frames a second, allowing more light into each frame, then took John Wayne aside and explained that, as he rose from his pew and exited down the aisle past the camera, he had to move in slow motion. When the film was projected at the standard twenty-four frames a second, he would appear to be moving normally. Wayne nodded obediently, understood perfectly. "I think we got it on the first take," remembered Earnest Day.
Only a director who had worked in silent films, where camera speeds were often adjusted for dramatic effect, would have known of this gambit; only Ford, in concert with an absolute professional like Wayne, could have tossed it off with such a casual, spur-of-the-moment brio.
Quote from Scott Eyman, Print the Legend: The Life and Times of John Ford (p. 402)