While Christie's novels are set contemporaneously with the time of writing (between the 1920s and 1970s), 1936 was chosen as the year in which to place the majority of Poirot episodes; references to events such as the Jarrow March were included to strengthen this chronology. With some exceptions, the series as a whole is set in roughly chronological order between 1935 and 1939, just prior to the Second World War. Numerous references in early episodes place the series primarily in 1935, progressing to 1936 by series four. Most references remain to 1936, moving slowly forward to 1937 by series eleven and 1938 by "Murder on the Orient Express". "The Big Four" is set explicitly in early 1939. The most notable exceptions to this chronology are "The Mysterious Affair at Styles", which narrates Poirot's first case in 1917, and "Curtain: Poirot's Last Case", which is set primarily in 1949. "The Chocolate Box" shows Poirot in the early 1900s, though the framing narrative remains consistent with the series' usual timeframe.

The opening titles were designed by Pat Gavin, and feature Art Deco–Cubist–style iconography, partly inspired by Cassandre, including images of Battersea Power Station, biplanes, boats, and a train with Poirot's name formed by the wheels.

The episodes aired from series 9 in 2003 featured a radical shift in tone from the previous series. The humour of the earlier series was downplayed with each episode being presented as serious drama and saw the introduction of gritty elements not present in the Christie stories being adapted. Recurrent motifs in the additions included drug use, sex, abortion, homosexuality, and a tendency toward more visceral imagery. The visual style of later episodes was correspondingly different: particularly, an overall darker tone; and austere modernist or Art Deco locations and decor, widely used earlier in the series, being largely dropped in favour of more lavish settings (epitomised by the re-imagining of Poirot's home as a larger, more lavish apartment).

The series logo was redesigned (the full opening title sequence had not been used since series 6 in 1996), and the main theme motif, though used often, was usually featured subtly and in sombre arrangements; this has been described as a consequence of the novels adapted being darker and more psychologically driven. However, a more upbeat string arrangement of the theme music is used for the end credits of "Hallowe'en Party", "The Clocks" and "Dead Man's Folly". In flashback scenes, later episodes also made extensive use of fisheye lens, distorted colours, and other visual effects.
Florin Court was used to represent Whitehaven Mansions.

Series 9–12 lack Hugh Fraser, Philip Jackson and Pauline Moran, who had appeared in the previous series (excepting series 4, where Moran is absent). Series 10 (2006) introduced Zoë Wanamaker as the eccentric crime novelist Ariadne Oliver and David Yelland as Poirot's dependable valet, George — a character that had been introduced in the early Poirot novels but was left out of the early adaptations to develop the character of Miss Lemon. The introduction of Wanamaker and Yelland's characters and the absence of the other characters is generally consistent with the stories on which the scripts were based. Hugh Fraser and David Yelland returned for two episodes of the final series ("The Big Four" and "Curtain"), with Philip Jackson and Pauline Moran returning for the adaptation of "The Big Four". Zoë Wanamaker also returned for the adaptations of "Elephants Can Remember" and "Dead Man's Folly".

Clive Exton adapted seven novels and fourteen short stories for the series, including "The ABC Murders" and "The Murder of Roger Ackroyd", which received mixed reviews from critics. Anthony Horowitz was another prolific writer for the series, adapting three novels and nine short stories, while Nick Dear adapted six novels. Comedian and novelist Mark Gatiss wrote three episodes and also guest-starred in the series, as have Peter Flannery and Kevin Elyot. Ian Hallard, who co-wrote the screenplay for "The Big Four" with Mark Gatiss, appears in the episode and also "Hallowe'en Party", which was scripted by Gatiss alone.

Florin Court in Charterhouse Square, London, was used as Poirot's London residence, Whitehaven Mansions. The final episode to be filmed was "Dead Man's Folly" in June 2013 on the Greenway Estate (which was Agatha Christie's home) broadcast on 30 October 2013. Most of the locations and buildings where the episodes were shot were given fictional names.