Kenneth Branagh’s Murder on the Orient Express must be at least the third adaptation I have seen of the famous novel. It seemed to me that, while the Belgian detective’s little quirks had been picked on and exaggerated by previous television efforts, all of the real charm of a Poirot mystery had been sacrificed to the construction of a flashier, rather incredible and perhaps more popular sort of character, a character who was not Hercule Poirot.

Nevetheless, I soon discovered that I was wrong. It was a tiny detail in Branagh’s performance that gave me hope as to the insight his new adaptation could offer into one of Christie’s masterpiece. For, all through the film, Branagh’s eyes had the right sparkle.

In the books, Poirot’s small eyes are of great importance. They are always full of expression shrewd and vigilant, and the light that animates them is often secret knowledge, a private joke.

This knowledge often corresponds with the solution of the crime at hand, and the joke is invariably on Hastings, or on whoever, including the reader, is witnessing Poirot’s display of genius.

There is indeed something incredibly satisfying in an Agatha Christie mystery, at least when one uses one’s ‘little grey cells’ and gets the solution right. John Curran, the editor of Christie’s notebooks, ascribes her long-lasting, boundless popularity to the fact that: “No other crime writer did it so well, so often or for so long; no one else matched her combination of readability, plotting, fairness and productivity”.

While all these elements are certainly true, and would be enough to grant any author eternal fame, however, they are not the only things that appeal to us in Christie.

They are the mechanics of her greatness, its sinews and bones – they are not its heart and soul. Similarly, one is usually given the impression that, behind the little sparkle in Poirot’s eyes, there is something more than the solution to a problem, something more than the frantic working of his ‘little grey cells’ and the serene application of ‘order an method’.

Indeed, all through his series of exploits, there is some greater, deeper secret ‘papa Poirot’ is in the knowledge of: that is the secret of understanding life and its power, as much as murder and its appeal.

There is a real appreciation of life in Poirot’s characters. It is something that goes beyond the mere insight in human nature that is essential for the solving of all Christie’s mysteries. When I first started reading Christie, I pledged my full devotion to Miss Marple over Poirot. Indeed, the extraordinary working of her mind is something she certainly shares with Poirot.

A crucial aspect, however, that differentiates Miss Marple from Poirot, is that while both revel in solving crime, the sweet old lady genuinely enjoys pointing her wrinkled finger towards those who have committed it; for Miss Marple, that justice will be served is as crucial as that the murder should be solved. Not so for Hercule Poirot, who believes in the existence of good and evil, but who also understands compassion.

This is not to say that Poirot would ever let a criminal go unpunished, but he does recognize that there are many more diverse layers to justice than Miss Marple would ever notice. In 4.50 from Paddington, the unyielding old lady regrets the fact that death penalty is no longer available for punishing the abominable murderer she has just exposed. By contrast, Poirot, more than once, mercifully allows his murders the shortcut of suicide. This ability to recognize the complexity of the world, to hate the murder and, at the same time, feel pity for the perpetrator, stems from his joie de vivre.

In An Autobiography, Christie writes: “Always when I woke up, I had the feeling which I am sure must be natural to all of us, a joy of being alive … there you are, you are alive, and you open your eyes and here is another day; another step, as it were, on your journey to an unknown place. That very exciting journey which is your life.” This true enjoyment of life is present as an undercurrent in all of Christie’s most enduring successes, and perhaps her greatest mystery.