Since 1989, there have been three James Bonds, six Doctors and at least five Sherlock Holmeses on screen. But there has been only one Hercule Poirot. Perhaps no other actor has claimed such ownership over a role they did not originally create as David Suchet, who tonight (8pm, ITV) starts his final quartet of adaptations of Agatha Christie's stories about the Belgian private detective.

When Curtain, the last episode, is shown - at a date undetermined, but likely to involve a major secular-religious festival - Suchet will have played Poirot in 70 episodes that cover all of the substantial fictions that were written about him. In a cleverly ominous prologue to tonight's episode - The Big Four - it seems at first that the sleuth has perished already.

Although it is common in music for a pianist to play, for example, a complete Beethoven cycle, and Simon Ruselll Beale has performed in radio dramatisations of all John Le Carre's George Smiley books, such completism is almost impossible in television because of the level of commitment required from both performers – who understandably fear typecasting – and network executives, who are prone to changes of mind and fashion: ITV is unrecognisable in personnel and structure from when the first Poirot was shown almost quarter of a century ago.

Suchet's versatility as an actor – and his careful spacing of the Poirots to average fewer than three a year – has prevented his becoming trapped by the character. He has been able to fit between the mysteries several major stage productions – such as The Merchant of Venice and Long Day's Journey into Night – and other TV appearances, including the lead role in a bio-drama about the ruined tycoon Robert Maxwell and another scandalous financier, Melmotte, in an adaptation of Trollope's The Way We Live Now.

But what has been most impressive about this Christie marathon is not just that he has done other work away from Poirot but that he has continued to do different things in Poirot. The great TV producer Tony Garnett (Cathy Come Home, Cardiac Arrest, Between the Lines, This Life) has argued that the historical weakness of much TV drama has been the lack of character development. Peter Falk's Columbo was essentially the same from pilot to finale; both because viewers are assumed to take comfort in familiarity and also, Garnett argues, so that the shows could be screened out of order on syndication channels.

In trying to maintain both his and the audience's interest through well over 100 hours of playing Poirot, Suchet faced two problems. One is that Christie, who became sick of her creation, did not allow the Belgian detective much space to change during her long immersion in his personality. The other is that the ITV series opted to smooth out the chronology of the novels and short stories – which were published over a period of 55 years – by locating most of the stories in circa 1936, although we have reached the later 30s for the final lot: tonight's The Big Four is set during the runup to the second world war.

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Suchet, though, has subtly aged the investigator, with greying moustaches and increasing stiffness in the skippy little walk he gave the detective. More importantly, there is an increasing sense of the accumulated weight of the cases on him. Although the plots are generally preposterous – tonight's is a gothic conspiracy thriller – the actor provides a centre of gravity, never letting go of the fact that, from the war to his work, Poirot has had too much connection with death. And the scripts for these final four stories are overhadowed, from start to finish, by the approach of his own funeral.

While the scripts often seem to burden him with interchangeable lines – such as , "I think that you lie!" and "You are in love with him, I think!" – Suchet drops little hints of the impact of the lonely bachelor life from which murder intermittently distracts Poirot. This famously fastidious actor seems born to play this notoriously pernickety man.

One of Suchet's greatest skills is listening, which is acknowledged as one of the hardest tasks in acting. Bad actors, drama teachers point out, can be seen waiting for the other speaker to stop and then start talking when they pause. But look at Suchet, when he is silent in a scene, and Poirot's face, eyes and mind are always alive, whether deeply concentrating or poker-facing a suspect or betraying his irritation at the deceit or indelicacy of what he is being told.

It seems unlikely that these circumstances will ever again come together – a prolific author, an actor always looking to perfect the portrayal, a TV company that found a banker in good financial times and then revived it in bad – to create another such marathon of screen acting craft.

Already a rare example of the use of the writerly possessive apostrophe in TV, the series should perhaps have added, for this final run, an actorly apostrophe: David Suchet's Agatha Christie's Poirot.