Like Arthur Conan Doyle with Sherlock Holmes, Christie was to become increasingly tired of her detective Poirot. In fact, by the end of the 1930s, Christie confided to her diary that she was finding Poirot “insufferable," and by the 1960s she felt that he was "an ego-centric creep." However, unlike Conan Doyle, Christie resisted the temptation to kill her detective off while he was still popular. She saw herself as an entertainer whose job was to produce what the public liked, and the public liked Poirot.

In contrast, Christie was fond of Miss Marple. However, it is interesting to note that the Belgian detective’s titles outnumber the Marple titles more than two to one. This is largely because Christie wrote numerous Poirot novels early in her career, while Murder at the Vicarage remained the sole Marple novel until the 1940s.

Christie never wrote a novel or short story featuring both Poirot and Miss Marple. In a recording, recently rediscovered and released in 2008, Christie revealed the reason for this: "Hercule Poirot, a complete egoist, would not like being taught his business or having suggestions made to him by an elderly spinster lady".

Poirot is the only fictional character to have been given an obituary in The New York Times, following the publication of Curtain in 1975.

Following the great success of Curtain, Christie gave permission for the release of Sleeping Murder sometime in 1976 but died in January 1976 before the book could be released. This may explain some of the inconsistencies compared to the rest of the Marple series — for example, Colonel Arthur Bantry, husband of Miss Marple's friend Dolly, is still alive and well in Sleeping Murder despite the fact he is noted as having died in books published earlier. It may be that Christie simply did not have time to revise the manuscript before she died. Miss Marple fared better than Poirot, since after solving the mystery in Sleeping Murder she returns home to her regular life in St. Mary Mead.

On an edition of Desert Island Discs in 2007, Brian Aldiss claimed that Agatha Christie told him that she wrote her books up to the last chapter and then decided who the most unlikely suspect was. She would then go back and make the necessary changes to "frame" that person. The evidence of Christie's working methods, as described by successive biographers, contradicts this claim.[citation needed]

Almost all of Agatha Christie’s books are whodunits, focusing on the British middle and upper classes. Usually, the detective either stumbles across the murder or is called upon by an old acquaintance, who is somehow involved. Gradually, the detective interrogates each suspect, examines the scene of the crime and makes a note of each clue, so readers can analyze it and be allowed a fair chance of solving the mystery themselves. Then, about halfway through, or sometimes even during the final act, one of the suspects usually dies, often because they have inadvertently deduced the killer's identity and need silencing. In a few of her novels, including Death Comes as the End and And Then There Were None, there are multiple victims. Finally, the detective organises a meeting of all the suspects and slowly denounces the guilty party, exposing several unrelated secrets along the way, sometimes over the course of thirty or so pages. The murders are often extremely ingenious, involving some convoluted piece of deception. Christie’s stories are also known for their taut atmosphere and strong psychological suspense, developed from the deliberately slow pace of her prose.

Twice, the murderer surprisingly turns out to be the unreliable narrator of the story.

In five stories, Christie allows the murderer to escape justice (and in the case of the last three, implicitly almost approves of their crimes); these are The Witness for the Prosecution, The Man in the Brown Suit, Murder on the Orient Express, Curtain and The Unexpected Guest. After the dénouement of Taken at the Flood, her sleuth Poirot has the guilty party arrested for the lesser crime of manslaughter. (When Christie adapted Witness into a stage play, she lengthened the ending so that the murderer was also killed.) There are also numerous instances where the killer is not brought to justice in the legal sense but instead dies (death usually being presented as a more 'sympathetic' outcome), for example Peril at End House, Death on the Nile, The Murder of Roger Ackroyd, Crooked House, Appointment with Death, The Hollow, and Secret Adversary. In some cases this is with the collusion of the detective involved. Five Little Pigs, and arguably Ordeal by Innocence, end with the question of whether formal justice will be done unresolved.