In Chapter 1, Joanna Southwood asks Linnet about whether the pearl necklace she's wearing is real. This scene is foreshadowing for the disappearance of the pearls later in the novel:
“I suppose these are real, aren’t they, Linnet?”
“Of course.”
“I know it’s ‘of course’ to you, my sweet, but it wouldn’t be to most people. Heavily cultured or even Woolworth! Darling, they really are incredible, so exquisitely matched. They must be worth the most fabulous sum!”
“Rather vulgar, you think?”
“No, not at all—just pure beauty. What are they worth?”
“About fifty thousand.”
Toward the end of the novel, the reader finds out later that Joanna is part of a jewelry stealing ring with Tim Allerton. When Poirot discovers Linnet's pearls have been replaced by a detailed replica, her murder looks motivated by burglary, but Poirot discovers that Tim quietly stole the real pearls without hurting Linnet.
In the first chapter, Joanna questions Linnet about the necklace in order to get a duplicate made, and she inquires about the cost to confirm that the pearls are worth stealing. She wants to make sure they're the most valuable kind of pearl possible. Cultured pearls, unlike Linnet's real pearls, are formed with human intervention (and therefore are commoner and less valuable). When Joanna mentions Woolworth, she's probably referring to Woolworths, an English department store that would have sold mass market clothing and accessories, including cheaper pearl necklaces.
In Chapter 1, Joanna describes Linnet's self-assurance, wealth, and beauty with a metaphor:
“Linnet Ridgeway, can you look me in the face and tell me of any one occasion on which you’ve failed to do exactly as you wanted?”
“Heaps of times.”
“Oh, yes, ‘heaps of times’—just like that—but no concrete example. And you simply can’t think up one, darling, however hard you try! The triumphal progress of Linnet Ridgeway in her golden car. […] In the meantime, enjoy the triumphal progress in the golden car. Only I wonder, I really do wonder, what will happen when you want to go down a street which has a board saying ‘No Thoroughfare.’”
The metaphor of Linnet's "triumphal progress in the golden car" illustrates Linnet's life. She has wealth, beauty, charm, and the energy to go after everything she wants. In this way, her life is like an expensive car: she can go anywhere with ease and style. Joanna then extends the metaphor: what will happen when Linnet cannot do as she wants? Joanna metaphorically speaks of this hypothetical impasse as a street marked No Thoroughfare, through which Linnet's golden car cannot drive.
This scene is also an instance foreshadowing because it is Linnet's determination to get what she wants, regardless of the boundaries she has to cross, that leads to her murder. Her betrayal of Jackie and engagement to Simon incites the events of the novel.
In Chapter 5, Jackie uses a metaphor to describe why Simon left her for Linnet:
There’s such a thing as glamour, Monsieur Poirot. And money helps that. Linnet had an ‘atmosphere,’ you see. She was the queen of a kingdom—the young princess—luxurious to her fingertips. […] Look at the moon up there. You see her very plainly, don’t you? She’s very real. But if the sun were to shine you wouldn’t be able to see her at all. It was rather like that. I was the moon . . . When the sun came out, Simon couldn’t see me anymore . . . He was dazzled. He couldn’t see anything but the sun—Linnet.
Jackie's first metaphor compares Linnet to a queen or a princess. Then, Jackie explains how Simon forgot about her so quickly with a metaphor, in which she compares herself to the moon and Linnet to the sun. Linnet outshone her, just as in the daytime the sun can make the moon impossible to see.
Later, in Chapter 6, Simon uses the same simile:
It’s like the moon when the sun comes out. You don’t know it’s there anymore. When once I’d met Linnet—Jackie didn’t exist.
This moment is foreshadowing, and Poirot notices it with suspicion—why do Simon and Jackie use the same metaphor? Perhaps Simon and Jackie decided upon this explanation for Simon's behavior when they were planning the murder.
In Chapter 5, Jackie uses a metaphor to describe why Simon left her for Linnet:
There’s such a thing as glamour, Monsieur Poirot. And money helps that. Linnet had an ‘atmosphere,’ you see. She was the queen of a kingdom—the young princess—luxurious to her fingertips. […] Look at the moon up there. You see her very plainly, don’t you? She’s very real. But if the sun were to shine you wouldn’t be able to see her at all. It was rather like that. I was the moon . . . When the sun came out, Simon couldn’t see me anymore . . . He was dazzled. He couldn’t see anything but the sun—Linnet.
Jackie's first metaphor compares Linnet to a queen or a princess. Then, Jackie explains how Simon forgot about her so quickly with a metaphor, in which she compares herself to the moon and Linnet to the sun. Linnet outshone her, just as in the daytime the sun can make the moon impossible to see.
Later, in Chapter 6, Simon uses the same simile:
It’s like the moon when the sun comes out. You don’t know it’s there anymore. When once I’d met Linnet—Jackie didn’t exist.
This moment is foreshadowing, and Poirot notices it with suspicion—why do Simon and Jackie use the same metaphor? Perhaps Simon and Jackie decided upon this explanation for Simon's behavior when they were planning the murder.
Hidden from view but loud enough to be heard, Simon talks to someone right after he finds out Jackie is on the boat. This moment, which occurs in Chapter 7, serves as foreshadowing for Simon's secret alliance with Jackie:
The two of them moved a little away. Without turning his head Poirot caught scraps of disjointed words: “. . . turn back . . . impossible . . . we could . . .” and then, slightly louder, Doyle’s voice, despairing but grim: “We can’t run away forever, Lin. We’ve got to go through with it now. . . .”
In this passage, Poirot overhears and presumes Simon is speaking to Linnet. This is a reasonable assumption to make—Simon says Linnet's nickname, after all. But Poirot claims in Chapter 29 that it was Jackie that Simon spoke to:
Then one night on this boat I thought I heard Simon and Linnet outside my cabin. He was saying, ‘We’ve got to go through with it now.’ It was Doyle all right, but it was to Jacqueline he was speaking.
Perhaps in the first scene, Simon used Linnet's nickname to cover up the fact that he was talking to Jackie, whom he was meant to be furious with. If that is true (and Poirot certainly doesn't seem to be wrong often), then this moment in Chapter 7 is foreshadowing. While Doyle's dialogue, "We've got to go through with it now," seems to an unsuspecting listener to refer to the boat ride and honeymoon, Doyle is, in fact, referring to the plan to murder Linnet.
Hidden from view but loud enough to be heard, Simon talks to someone right after he finds out Jackie is on the boat. This moment, which occurs in Chapter 7, serves as foreshadowing for Simon's secret alliance with Jackie:
The two of them moved a little away. Without turning his head Poirot caught scraps of disjointed words: “. . . turn back . . . impossible . . . we could . . .” and then, slightly louder, Doyle’s voice, despairing but grim: “We can’t run away forever, Lin. We’ve got to go through with it now. . . .”
In this passage, Poirot overhears and presumes Simon is speaking to Linnet. This is a reasonable assumption to make—Simon says Linnet's nickname, after all. But Poirot claims in Chapter 29 that it was Jackie that Simon spoke to:
Then one night on this boat I thought I heard Simon and Linnet outside my cabin. He was saying, ‘We’ve got to go through with it now.’ It was Doyle all right, but it was to Jacqueline he was speaking.
Perhaps in the first scene, Simon used Linnet's nickname to cover up the fact that he was talking to Jackie, whom he was meant to be furious with. If that is true (and Poirot certainly doesn't seem to be wrong often), then this moment in Chapter 7 is foreshadowing. While Doyle's dialogue, "We've got to go through with it now," seems to an unsuspecting listener to refer to the boat ride and honeymoon, Doyle is, in fact, referring to the plan to murder Linnet.