Agatha Christie’s reputation as The Queen of Crime was built by the large number of classic tropes that she introduced, or for which she provided the most famous example. Christie built these tropes into what is now considered classic mystery structure: a murder is committed, there are multiple suspects who are all concealing secrets, and the detective gradually uncovers these secrets over the course of the story, discovering the most shocking twists towards the end. At the end, in a Christie hallmark, the detective usually gathers the surviving suspects into one room, explains the course of his or her deductive reasoning, and announces the guilty party. Beware that this article contains numerous plot spoilers.
This is a very common clue to the reader that something specific should be regarded as relevant in the immediate events. In The Mystery of the Spanish Chest, a character seems to remember that there was something odd about a room. Poirot remembers that she is puzzled, and later prompts her to remember that a screen was in the wrong place. This is also used to heighten suspense as to whether the person concerned will "remember" the crucial item, or, having remembered it, whether they can communicate with the detective before being silenced, as for example in After the Funeral, where Helen Abernethie is attacked while trying to telephone through information about 'something odd' she has remembered which gives a clear clue as to the killer.
In many other examples, a person appears familiar for some reason. In her early novels, Christie sometimes uses this to indicate that the person is someone else in disguise, but later (when her plot machinery is less incredible) the reason for the familiarity is more subtle. Familiarity, especially in the eyes, is often used to foreshadow illegitimate children or hidden family relations, as in Hercule Poirot's Christmas.
In Murder on the Links, Poirot draws the attention of Hastings to footprints in one of two flower beds. Hastings is misled into thinking that Poirot is interested in the footprints, but he is actually interested in their absence from the other bed, where they should have also been found.
This trope – which appears in several different forms throughout her novels – was borrowed by Christie from Arthur Conan Doyle’s short story, Silver Blaze. In this, Sherlock Holmes refers to “the curious incident of the dog in the night-time”, where the “curious” thing is the fact that the dog does not bark rather than that it does. Christie effectively admits the debt in the tenth chapter of Cards on the Table when her crime novelist character, Ariadne Oliver, explicitly mentions the source. The same reference is also specifically mentioned by Poirot in Murder in the Mews.
In the first chapter of Lord Edgware Dies, Hastings tells the reader that Poirot has always attributed his solution of this mystery to “a chance remark of a stranger in the street”. (The remark – “If they had just had the sense to ask Ellis right away” – has nothing directly to do with the mystery.) This is just one of many examples when the nature of the mystery is explained by an epiphany in which the detective makes a relevant discovery on the strength of a random occurrence.
In Murder on the Links, most of the confusing elements of the crime are discovered to have been part of an elaborate plan by the victim to stage his own death and disappear. It is when he is happened upon by the real murderer that the final elements are added to the puzzle.
Similarly, in The Mystery of the Spanish Chest the victim himself plans to hide in the chest and catch his wife with the man that he suspects of being her lover. The murderer kills him while he is in the chest, resulting in a more complex situation to be solved than might otherwise have arisen.
In Murder on the Links, Poirot stresses the potential importance of a length of lead pipe that is completely overlooked by a rival detective who only focuses on very small clues.
In a sense, many of Christie’s novels employ the same trope on a different level, in the sense that the murderer is rarely “the person one would least suspect”: more usually he or she is a character that has been very visible from early in the novel.
Another contentious trope used repeatedly in Christie’s work is the concealment of identity. In Third Girl, Taken at the Flood, A Murder is Announced, Hercule Poirot's Christmas and After the Funeral characters are able to pass themselves off as relatives who have been unseen for considerable periods. In Elephants Can Remember, the identity of the murderer is switched with that of the victim at the victim's own request.
More incredibly, in Third Girl a young woman fails to notice that her stepmother is also living with her in disguise as a flatmate and, in Murder in Mesopotamia, a woman marries a man without realising that it is actually her former husband. In Why Didn't They Ask Evans?, a man well-known to all the major characters is able to disguise himself as another man equally well-known to the same characters without difficulty until given away by having differently shaped ears.
Impersonation may be for the specific reason of making the character appear to do something specific (as in After the Funeral, where Cora Lansquenet makes a contentious remark about her brother's death, or the short story The Dream, where a man impersonates a millionaire to make him appear suicidal) or to establish an alibi, as in Evil Under the Sun, and the short story Four and Twenty Blackbirds. A variation on this theme is where someone adopts a secondary identity in order to allow their real identity to disappear mysteriously by 'redirecting attention'. This trope is used in At the Bells and Motley from The Mysterious Mr Quin, and in modified form in Dead Man's Folly. In Murder on the Orient Express, identities are concealed so they will not seem to hold a grudge against the victim, which they all do have. One is even a celebrity in disguise.
In A Murder is Announced the silly and forgetful Dora Bunner tells Inspector Craddock what one particular character was doing shortly before the murder took place. But because she is so unreliable, everybody believes she was mistaken until she started to believe the version of the murderer herself. In The Mousetrap, Mrs Boyle points out that one character cannot be who he pretends to be, but nobody pays attention since Mrs Boyle is presented as a rather unpleasant woman who complains about everyone. In Crooked House, Brenda Leonides tells the narrator pretty early in the book that she thinks the character, who later turns out to be the murderer, might not be quite right in the head, but nobody believes her since Brenda herself is the main suspect in the poisoning of her much older and rich husband.
In several stories, the criminal plays with time, to make it look as though the crime took place when the criminal was elsewhere. In Evil Under the Sun the criminals fake a murder for a time when they both have alibis, then commit it later while the preliminary investigative bustle distracts attention. In Hercule Poirot's Christmas the murder is committed an hour before it appears to have taken place, at a time when the criminal is elsewhere in front of witnesses. In Hickory Dickory Dock, a criminal's accomplice makes a phone call that is ostensibly from the victim, at a time when the criminal is standing in front of Poirot. In The Plymouth Express and The Mystery of the Blue Train (the former being shortened version of the latter) the criminal disguises herself as the victim at a train station to create the impression that the victim was still alive when, in fact, she had been killed earlier. Playing with time invariably involves devices such as fake phone calls, gunshots, screams, disguises, people pretending to be dead, and other devices that take advantage of an observer's assumptions.
Christie is famous for her climactic scenes in which the detective brings together all the surviving suspects and explains the solution to the mystery. Frequently the guilty party breaks down and makes a dramatic confession under pressure. The announcement is invariably long-winded, first discussing the clues and the detective's line of reasoning, before the solution is explained. This is an artificial device but works to increase suspense and create greater reader engagement with the problem.
In Peril at End House, a young woman (Nick Buckley) appears to be the target for a number of murder attempts. In fact she has arranged these in order to mask her own murder (of a distant cousin, Maggie) as another botched murder attempt that has miscarried.
In The Murder of Roger Ackroyd, the murder has been committed by the narrator, Dr. Sheppard, who never lies but omits mention of any of the actions that would demonstrate his guilt. The same thing happened in Endless Night.
In Hercule Poirot's Christmas, the murder has been committed by one of the investigating policemen, who also happens to be the illegitimate son of the victim. In the short story The Man in the Mist, the investigating policeman, who is also the victim's husband of twenty years ago, commits the murder.
In Death on the Nile the initial suspect, Jacqueline de Bellefort, has actually shot the victim’s husband, Simon Doyle, before the murder. The solution to the mystery reveals that they are working together, and shooting has been carefully staged. Similarly, the conspirator cousins in The Mysterious Affair at Styles only pretend to hate one another. A husband and wife, despite appearances that their marriage is breaking down, team up to commit the murder in Evil Under the Sun. The same device underpins Endless Night.
While it is a common red herring to include unrelated minor crimes like robberies in the stories, in Cat Among the Pigeons two murders actually have no connection at all except for place and method. The second murderer just happened to mimic the first murder in execution.
In Evil Under the Sun, the body of the victim is apparently discovered by two characters, one of whom goes to fetch the police. The murderer, however, has only “discovered” the body of his accomplice, and is left free to murder the real victim with a seemingly perfect alibi established. In Cards on the Table, the murderer finds his victim sleeping, tells the maid he's dead then kills him during the resulting confusion.
In The Hollow, Poirot arrives at the scene of a murder in time to see a woman with a gun in her hand standing over the body of her husband, who is bleeding to death from a fresh bullet wound. It turns out at the end of the novel that she did in fact shoot him, but that this fact has subsequently been obfuscated by the other witnesses, all incriminating themselves to exonerate the woman for her perhaps justifiable act. Conversely, in Hickory Dickory Dock, the murderer is incriminated by so many clues that it appears he is being framed, with a lack of obvious motive and a clever false alibi for one of the murders weighing in his favor until the end. In Lord Edgware Dies, the murderer announces how she would kill the victim, and when doing it announces herself at the door of the victim's house perfectly truthfully, but has arranged apparent alibis to make it seem that she was framed. In The Murder at the Vicarage, the murderers each confess separately, but are cleared and only much later proved to be in fact guilty.
A variation on this is in Ordeal by Innocence, where the man found guilty for the crime, whose posthumously revealed alibi prompts a reopening of the case, turns out to have arranged the murder after all, though not committed it by his own hand.
In some stories, such as Murder In The Mews, a suicide or accident proves to be exactly that, but someone comes along later and rearranges the scene in order to incriminate someone else. In a few stories, such as The Labours of Hercules, someone who is thinking about committing murder is prevented from going any further.