The road is seldom clear in noir, and Al Roberts fumbles though the confusion like it's an unfamiliar road, like the "detour" of the film's name. But Roberts doesn't seem to be doing the driving; it seems fate has taken the wheel and he is just a hitchhiker along for the ride. At first he struggles against it, but by the end he adopts a fatalistic attitude.
"I keep trying to forget what happened and wonder what my life might have been like if that car of Haskel's hadn't stopped. But one thing I don't have to wonder about, I know. Someday a car will stop to pick me up that I never thumbed. Yes, Fate or some mysterious force can put the finger on you or me, for no good reason at all." - Al Roberts
Much of "Detour" is framed by a diner scene where Roberts narrates the story, and these scenes are shot in classic noir style. The lighting is shuttered to primarily light Roberts' eyes, both drawing focus to the agony in his expression and creating a feeling of claustrophobia, a feeling echoed by cramped scenes in the car and the apartment he rents with the tough femme fatale, Vera. In these diner scenes, Roberts is isolated from those around him; activity continues all around, but the other characters are in a dim light and the shadows seem to wall them off. The camera zooms in on shots of Roberts' coffee cup, and we can almost taste the dregs at the bottom of it; a juke box, playing the song that spurs Roberts' tale, fills the frame as its music segues into the orchestra music from Roberts' past and the start of his story.
Edgar G. Ulmer got his start in film during the German Expressionism period of the 1920s where producers used abstract settings, lighting and shadow in place of lavish production budgets. So although Ulmer was accustomed to dealing with budget constraints, the fact "Detour" was filmed with one of the lowest budgets of its day makes the film's attention to style all the more remarkable.