John Garfield & Lana Turner: struggling to stay afloat in 'The Postman Always Rings Twice'

Frank: That other girl. She don't mean anything to me.
Cora: She told me you were going away with her.
Frank: Why didn't I go away and never come back? Cause we're chained to each other, Cora.
Cora: Don't tell me you love me.
Frank: I do.
Cora: Oh, but love wouldn't mean a thing to me.
Frank: Do you hate me?
Cora: I don't know. But we've got to tell the truth for once in our lives.

Cora: Oh Frank, I couldn't turn you into Sackett. I couldn't have this baby and then have it find out that I'd sent its father into that poison gas chamber for murder.
Frank: Was the baby the only reason?
Cora: No. Oh Frank, please, there's one thing I have to be sure of. No, don't ask me any questions. Just take me down to the beach.

The smutty grit in the novel, with its violent sex between lovers so enflamed with shameful desire that they actually vomit, is replaced in the film version by sparkling white sets and romantic clinches.

"Some elements of 'The Postman Always Rings Twice' sprang from real life, notably the infamous Ruth Snyder murder case, in which the defendant murdered her husband with the help of her lover, who she then tried to poison. Cora was modeled on a girl who pumped Cain's gas at a service station. She was kind of cheap, Cain acknowledged (more like the Cora of the novel than the Cora of the motion picture, who just seemed too classy to be working in a cheap roadside diner), but so sexy that she stuck in his memory". Source: bernardschopen.tripod.com

When they are way out in the deep dark ocean and Cora tiredly struggles to stay afloat, she asks for him to either save her from drowning and pledge to restore their love, or leave her to perish. Frank rescues his exhausted lover.

Cora: What I wanted to be sure of was, whether you trust me, if you don't believe that I can ever turn on you again. I'm too tired. I could never make it alone. Nobody'll ever know.
Frank: Cora, Cora. Don't say another word, darling. Save your strength. I'll take you in. (they reach the shore and the car) Are you sure now?
Cora: I'm sure.

Lana Turner as Cora Smith in 'The Postman Aways Rings Twice'

James M. Cain worked as an insurance salesman, like Walter Neff in 'Double Indemnity', and thus he was familiar with the schemes used by the insured trying to swindle money. Women would often take out accident policies against their husband, without his knowledge —like Phyllis Dietrichson did in 'Double Indemnity'— and would have the postman ring twice when insurance documents arrived, so she could keep this mail unknown to her husband. So the postman ringing twice became a sort of symbol of a wife's duplicity, often linked to a sexual betrayal.