How many stirring moments does it take to make a great movie? Whatever the number, “The Rider” has more than enough—moments between a man and a horse; between an injured man and his shattered friend, or his different-minded sister; silent moments when the camera studies the hero’s appealing face, and one heart-stopping fraction of a moment when a father tells his son to whistle. This poetic, laconic and ineffably beautiful drama has an unerring feel for its subject, a young cowboy struggling against his implacable fate in the American West. That’s notable in itself, and all the more so since the film was written and...