'Five Little Pigs' is a Poirot novel and spoilers abound here, so if you haven't read or seen it yet and you want to be surprised, turn away from this post now.
I haven't read the source novel for this, but have seen the 2003 TV adaptation, directed by Paul Unwin as part of the Agatha Christie's Poirot series. This is one of the later episodes, which tend to stray further from the plot of the novel than the 1990s episodes did, not just adding or subtracting characters but changing motivations and, most often, altering various suspects' sexuality (homosexuality is not entirely absent from Agatha Christie's works - if you haven't seen the more faithful 1985 Joan Hickson adapation of A Murder is Announced, do, Paola Dionisotti's performance is heart-breaking - but it's more subtle, since it was illegal at the time).
As far as I know, however, the method of the murder, which is the bit that's of particular interest to classicists, is the same as that in the book. This mystery is one of the 'cold case' ones, in which a young woman asks Poirot to establish whether or not her mother committed the murder she was executed for, and Poirot hears five different versions of the murder from the five main witnesses.
The method of murder is signposted early on, as, at young Angela's urging, Plato's account of the death of Socrates is read out. Socrates was executed by being ordered to swallow hemlock, a poison which kills relatively quickly, but slowly enough that the victim can sense what is happening. Until I did Classics, I only knew of the death of Socrates from the Horrible Histories; The Groovy Greeks, which has a rather good translation/retelling of the passage. Hemlock makes you go slowly numb from the feet upwards, until it reaches your heart and kills you. I've always thought there was something rather creepy about it. Socrates knew full well what was happening, but of course, that's what's creepy about it, the way Plato records him having a perfectly lucid conversation while his body slowly fails from the ground up. (Among his last words, according to Plato were 'I owe a cock to Asclepius' - whether this is ironic, since Asclepius is god of healing, or put in by Plato to demonstrate that Socrates, executed for corrupting the youth and turning them against the gods, was in fact a religious man, I'm not sure. Probably a bit of both).
Anyway, in Poirot, it's doubly creepy, as Amyas the artist only realises what is happening to him as his body slowly goes numb. The TV adaptation features a recurring shot - appearing in several of the flashback sequences that relate what happened that day - of Amyas turning towards the camera, clinging to his easel for support and looking distressed - drunk, as the others thought at the time. In fact, as Poirot realises, he had already been poisoned and was clinging to his easel because his legs had already gone numb, and the poison (derived from hemlock) had also had enough effect to prevent him from crying out. It's absolutely the stuff of nightmares - Amyas knows he's dying, but can neither move nor cry out, while his friends look on from a distance and his murderer watches and smiles.
The whole thing is beautifully shot and particularly creepy, suffused with a warm yellow-orange light for the flashback sequences, and the cameraman appears to have borrowed the Vaseline from the cameramen from orignal series Star Trek (you know, the Vaseline they used to smear all over the camera whenever a beautiful woman got a shot to herself). The production values are excellent and the cumulative effect adds to the incredible creepiness of Amyas' death and tragedy of his wife's.
The girl, Angela, who was so interested in Socrates grows up to be an archaeologist, specialising in the archaeology of the ancient Near East, by the look of the part of the British Museum she's in when she gives a lecture in the TV version. Agatha Christie herself accompanied her second husband on digs in Mesopotamia - hence she wrote Murder in Mesopotamia - so it is unsurprising that she gave this career to a character interested in ancient history (a character permanently scarred by an earlier incident - but it's probably best not to try to psychoanalyse Agatha Christie on the basis of a novel).
Hemlock, by the way, is also the method of murder-or-mercy-killing-I-can't-quite-decide-which/suicide in Robin and Marion, a very good film that happened to be on telly the other week, Sean Connery and Audrey Hepburn at their best.
The reading of the death of Socrates acts as a combination of foreshadowing and clue, and heightens the sense of tragedy around Amyas' (and Caroline's) death - since we feel for Socrates and his undeserved death, we feel for them too. A sense of tragedy is something that's often missing from Agatha Christie, and it's refreshing and really interesting to see it employed here, in this case that revolves around a wrongful execution.
(By the way, did everyone see Doctor Who this week - 'Vincent and the Doctor'? Very good, I wept buckets...)