In Chapter 15, Poirot uses metaphors to describe a possible motive that might have led Pennington to kill Linnet:
If Linnet Doyle were to die, her fortune would pass to her husband—and he would be easy to deal with; he would be a child in the hands of an astute man like Andrew Pennington. Mon cher Colonel, I tell you I saw the thought pass through Andrew Pennington’s head. ‘If only it were Doyle I had got to deal with . . .’ That is what he was thinking.
Poirot metaphorically says that Simon would be so financially uninformed as to be "a child" in Pennington's control. Poirot's phrase "in the hands of an astute man like Andrew Pennington" is also metaphorical language, since Pennington would not literally hold Simon. Nor could Poirot have literally seen "the thought pass through Andrew Pennington's head," but he uses this metaphor to mean that Pennington's intentions and logic were transparent to him.
Poirot speaks excellent English, but his French phrases ("mon cher") and deliberate manner of talking, free from the Britishisms and frequent contractions of other characters, remind the reader that he is French. His distinctive manner of speaking perhaps even helps the reader imagine him talking in a French accent.
In Chapter 19, Jackie frets over Simon's leg after shooting him. Oddly enough, Simon forgives her quickly, downplaying the injury with an idiom, colloquialisms, and his British dialect:
[Simon:] “Not you. Not with a rotten little peashooter like that. . . .”
[Jackie:] “And your leg! Perhaps you’ll never walk again. . . .”
“Now, look here, Jackie, don’t be maudlin. As soon as we get to Aswan they’re going to put the X-ray to work, and dig out that tin-pot bullet, and everything will be as right as rain.”
A peashooter is a toy gun that peas (or other small objects) can be shot through. By calling her gun a peashooter, Simon claims that Jackie's gun is too small to do any damage. "Tin-pot bullet" suggests, like the peashooter comment, that the bullet was weak and ineffective like a homemade bullet crafted in a tin pot. While it's not a literary device, readers should know that the uncommon word "maudlin" means "easily moved to tears."
"Right as rain" is an English idiom meaning something is okay or correct—or, more specifically, that someone will feel well again after having felt bad. While it's not clear what this specific idiom refers to, "right as [object]" was a common idiom format dating from the 1600s onward. It's likely that "right as rain" is the variation that stuck around because it's alliterative. Simon's use of the idiom and colloquialisms such as "peashooter" emphasizes his Britishness and commonness as well as his anxiety to reassure Jackie. His downplaying of the damage to his leg might clue the reader into his secret plot with Jackie.