[Spoiler] Were Jamie and Darby...
... even married? I'm thinking of what Darby said about hitchhikers - that people wouldn't pick up a guy hitch-hiker, but if he's with a girl, they will... she seemed to imply that she was with Jamie so that he appeared 'normal' and unthreatening when he approached Josh.
Also, she seemed to just pack up and disappear towards the end of the film, which maybe also indicates that they were never married.
Just can't remember whether the subject was covered (or if I missed it)