The question is posed:
"U-Turn is the latest variant on the basic story of The Postman Always
Rings Twice.  Does anybody know if Postman is more or less the origin of
this drifter meets doublecrossing couple plot?  (I mean the book since
none of the film versions are anywhere near as good.)"
 
The drifter as the driving force behind a story has to be ancient. The
traveller as the audience for, and link among  a series of tales is used
in Chaucer's Canterbury Tales. R.L. Stevenson's "The Body-Snatcher"
relies on the device. Melville's "Moby Dick" used Ishmael, drifter as
actor, narrator, and audience.
 
The genius of "Postman" lies (arguably) not in its use of the drifter
image, but in the fact that it is a retelling of the story of Adam and
Eve and the (by now infamous) apple. Two lovers, too eager and smart for
their own good, win a prize they should not have, and are destroyed by
it. They are clever enough to avoid the immediate (but not the ultimate)
consequences of "immorality".
 
"Payment Deferred" a C.S. Forester novel made into a film, makes the same
point. A clever killer avoids conviction
 for his crime but is convicted and put to death for one he did not
commit. Viewed in this light, the Book of Genesis and/or C.S. Forester,
are the precursors of  "Postman".
 
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Screen-L is sponsored by the Telecommunication & Film Dept., the
University of Alabama.