Long before Denzel Washington opens his mouth to explain, the meaning of the title “The Little Things” is clear enough: Pay close attention, because details matter. I should admit that I missed a few at the very beginning. This fastidiously grim psychological thriller opens on a dark stretch of California highway, illuminated only by a young woman’s headlights and then by the lights of the car that speeds up behind her, then roars ahead before coming to a sudden, disquieting stop. So swiftly does the writer-director John Lee Hancock create a sense of menace that the significance of some of those little things the B-52’s “Roam” blaring from the car radio, the conspicuous absence of a cellphone might elude you, too, until after you’ve finally exhaled.