The classic Film Noir (e.g. Murder, My Sweet) revolves around a hero whose attention is divided between a plain, brunette "good girl" and a glamorous blonde femme fatale. Detour, in contrast features a good girl who is a blonde and glamorous singer. The femme fatale is a "good girl gone wrong" whom he describes as possessing, "a beauty almost homely, it's so real." This reverses the typical situation: Vera's character, founded on the "good girl" stereotype but exhibiting the behaviour of the femme fatale, implies that all women pose a threat to men and male dominance.
An important aspect of Detour, and one without which no film noir would be complete, is the presence of the femme fatale (Vera). Her character differs significantly from the traditional fatal woman of A-features such as Murder, My Sweet and The Maltese Falcon. One reason for this is a resurgence of economic determinism. Vera is just as destitute as the hero (Al). She has been described as a "skid row femme fatale." This is somehow more realistic and less romantic than the glamorous sirens of fashion-conscious A-features. Accordingly, Vera inverts the traditional femme fatale role.
The revelation of the femme fatale's moral turpitude is also inverted. Taking Murder, My Sweet's Mrs. Grayle as archetypal, the femme fatale's usual method of subjecting the hero to her will is to seduce and flatter him until a relationship is formed on the basis of sexual addition and misplaced trust. It is only later the she is glimpsed in her true light as manipulative and sadistic. Vera is quite different. She is immediately and openly hostile to Al and never relents, never believes his innocence. This disparity with A-features makes Vera seem like a harsh dose of reality in contrast to a romanticised stock-character. At least Phyllis Dietrichson's actions were intended to improve her future, and may be aligned with optimistic concepts of the American dream and individual enterprise. Vera has no future - she is dying of a fatal illness - and her imprisoning of Al is motivated by a more vindictive spirit, a sadistic impulse. She has nothing to gain but doesn't want to suffer alone.
Detour depicts this anxiety in extremis. Unlike the A-feature hero (Spade and Marlowe) Al's identity is not merely threatened by the femme fatale and finally wrested from her clutches (as in The Maltese Falcon). Al literally loses his identity, shedding it to take temporary refuge in Haskell's. This is symbolised by exchanging his clothes for Haskell's, but results in Al's arrest because he has also acquired his past sins, giving a dark, haunted tone to his impersonation.
