Unmarked spoilers are ahead, per Spoilers Off rule.
The novel:
The 1982 film:
Fridge brilliance
The fact that Patrick Redfern is a Mr. Fanservice whose tiny swim briefs contrast with the period one-piece bathing suits worn by the other men may be a subtle hint that he is not the gullible victim of a man-eater, but The Casanova who is expertly using his good looks to seduce the foolish Arlena for murderous purposes.
A related one for Christine, who makes a point of telling Poirot that she can't go out in the sun as her skin blisters, and wears an excessively billowy outfit covering her whole body when she goes sketching at the beach. The fact that she turns up at the outdoor tennis court wearing a rather small athletic dress that leaves her limbs exposed is a hint to the viewer that she was lying about her skin.
