Donna: Agatha, people love your books, they really do, they’re gonna be reading them for years to come. Agatha: If only! Try as I might it’s hardly great literature, now that’s beyond me. I’m afraid my books will be forgotten, like ephemera.
The Queen of the classic mystery, ranked with Arthur Conan Doyle as the greatest mystery writer of all time. Her stories are elaborately plotted puzzle pieces, full of false identities and faked deaths. She enjoyed a very long career; her first novel, The Mysterious Affair at Styles, was published in 1920, while her final novel, Sleeping Murder, was published posthumously in 1976. Among the best-selling authors of all timenote The Guinness Book says she's sold more books than any other individual author. If you count shorter works, William Shakespeare takes the lead. If corporations are invited, the collected works of Walt Disney Productions top the list. Regardless, she's sold three billion copies in over 100 languages.Her principal detectives were:
Hercule Poirot, a retired Belgian police detective turned P.I. Fastidiously neat, he pretended to be a Funny Foreigner in order to put his clients and suspects off their guard. Agatha Christie herself eventually tired of the character, but since fans enjoyed him, she continued to write Poirot stories. He appeared in 33 novels and 51 short stories.
Miss Jane Marple. Seemingly a fluffy old spinster, her mind was as sharp as her knitting needles; having lived in small towns her whole life, nothing about human nature ever surprised her. She appeared in 12 novels and 20 short stories.
Tommy and Tuppence Beresford, a husband and wife team. Their adventures were more like spy novels than straight mysteries, though they did contain elements of classic detective novels. They appeared in five books: four novels and a short story collection, Partners in Crime.
Christie also wrote 16 novels which did not feature any of her series detectives. These ranged from traditional mysteries with one-shot detectives to Thrillers which placed more emphasis on action than detection. The latter were almost universally the most poorly received of Christie's works, while one of the former, And Then There Were None, is widely regarded as one of her best, even the best.While most of the stories are nominally set in the year of publication, in practice they all take place in the time of the Genteel Interbellum Setting. Of Christie's series detectives, only the Beresfords age in real time. Poirot and Marple both begin as elderly characters and over the course of Christie's 56-year career, age roughly 20 years at most. (This matches a line from the final Poirot novel Curtain, in which it is stated that it has been "over twenty years" since the first adventure, The Mysterious Affair at Styles, which came out in 1920 and presumably was set in the present day.)Christie possessed an uncanny ability to subvert the reader's expectations. Being well aware of the mystery conventions of the time, she was frequently able to subvert them for a Twist Ending. For example, Christie knew that there were certain characters, who by virtue of their role in the story, the reader would not suspect. To drive home the point that the reader should suspecteverybody, she would frequently make one of these characters the murderer.
Adaptation Decay: She parodied this in her novel Mrs McGinty's Dead, where her Author Avatar, Adriane Oliver, works on a theatre adaptation of her book, and she complains that the characters are completely changed.
Ironically, in real life, Agatha Christie's main complaint about early stage adaptations of her plays was that they stuck too closely to the books, as she felt that a murder mystery should surprise people.
Interestingly subverted in Evil Under the Sun: while the victim is disruptive in the community and has personality issues, to put it mildly, the worst of her actions are being being carefully staged by the killer and his accomplice. Poirot has already realised that her addiction to sex/romance/drama makes her vulnerable to manipulation and exploitation, not liable to perform it on others — she's not intelligent enough.
Another subversion in Sleeping Murder: despite what we're led to believe, the victim wasn't a slut or a terrible wife. The only person who has led us to believe this — the only person left who knew her really well — has been misleading us because he killed her (and he's not exactly the picture of sanity himself — his lies probably come from the fact that he may have lusted after her and certainly wanted her all to himself... but he happened to be her own brother.)
Subverted in Five Little Pigs: Caroline Crale was convicted of murdering her husband Amyas who was having an affair with his model - one of many. It turns out he never intended to leave Caroline, and he was murdered by said model.
Notably averted in Towards Zero too - the victim is a good, caring and hospitable (if a tad old-fashioned) person, and her death serves as Moral Event Horizon in-universe, which is actually the motive of her murder all along.
Author Appeal: Christie's second husband was an archaeologist, and Christie eventually developed a strong interest in the subject herself. Not surprisingly, several of her mid-period novels reflect this interest either by taking place at an archaeological dig (Murder in Mesopotamia) or by simply having such a setting as the background (Appointment with Death). This interest eventually led Christie to actually set one novel in ancient Egypt itself (Death Comes as the End).
In Cards on the Table (1937) Ariadne Oliver is the author of The Body in the Library. Christie's real novel of the same name would be published in 1942.
Author Filibuster: Christie novels tended to have long Start to Corpse times, something which she was occasionally criticized for. She used the first chapter of Towards Zero to respond to these criticisms by having a character deliver a lengthy speech on how a murder is the culmination of a murderer's plot rather than the instigating point, and thus should come as late in the book as possible.
Death Comes as the End: Sobek, the second son, is a hard-drinking womanizer who wastes money on expensive dancing girls.
A Pocket Full of Rye: Lancelot Fortescue, who was turned out of the family business and moved to Kenya.
Bowdlerise: And Then There Were None was originally titled Ten Little Niggers, then retitled Ten Little Indians, then retitled yet again when that title was recognised as offensive.
The French edition kept the original title - translated in French obviously - ( Dix petits nčgres)
Quite a few foreign editions still use the old title actually. The Russian edition is worth mentioning as well since it uses the original title for its movie adaptation (which is the only one not to Bowdlerise the plot of the book in any way)
Brother-Sister Incest: The killer in Sleeping Murder turns out to be the victim's brother, whose obsessive overprotectiveness of his sister is strongly implied to contain an element of lust for her.
Bunny-Ears Lawyer: Poirot's many eccentricities are compensated by his exceptional skills as a detective.
Busman's Holiday: Poirot, all the time, and even Marple occasionally. The lesser-known Parker Pyne suffered from it even more than Poirot did.
The Butler Did It: Several times, in different variations, both straight and skewed (for instance: the butler did it, but he wasn't really a butler).
Cannot Spit It Out : If the "secondary victim" was not a blackmailer, it was usually The Ditz, ( typically low-class and female), who knew something but was too dumb to understand what they knew. They would usually only hint to protagonists and then get murdered when the hints got to the murderer.
Nicely subverted in one book, where a dumb parlourmaid lives to tell what she knows, but another person who didn't know anything and bluffed is killed.
Canon Welding: Christie's main detectives never crossed over, but several minor characters did, effectively tying the majority of her detective fiction into the same universe. See the article on this trope for more information.
Celibate Hero: Although Poirot never has a romantic/sexual relationship of any kind, he is more a Celibate Hero of the "Love is a Distraction" variety than a true asexual. He typically acts gallant towards the women he meets — much more so than, say, Sherlock Holmes in similar situations — and he often makes polite comments about their looks and/or fashion choices; Hastings even jokingly remarks in Curtain: Poirot's last case that the detective prefers showy, voluptuous redheads. It never goes beyond that, though.
Note also that Poirot is portrayed as a devout Catholic, so it makes sense that he would avoid sex without marriage.
Poirot does seem to be attracted to Countess Vera Rossakoff. However, their relationship never develops beyond a mild flirtation.
He does privately reflect more than once on how unattractive he finds the post-WW1 woman, with their (to his 19th century eyes at least) rather boyish style of dress and behaviour
There's also Miss Marple, who has never been married, and whose love life, past or present, is rarely ever discussed. In A Caribbean Mystery, in discussing a girl who was encouraged to date a young man her parents approved of (after she broke up with an objectionable boyfriend), Miss Marple reflects on a young man she once dated who seemed very exciting, until he was warmly welcomed by her father. (She found him very boring after that.)
It might be interesting to bear in mind that "dating" didn't always carry the same connotations in the Victorian England Marple grew up in, and that the full and literal meaning of "old maid" (a term repeatedly used to describe Marple) is "old (female) virgin".
Cerebus Rollercoaster: Christie wrote her most serious and psychological works (like Five Little Pigs) in the middle of her career, in the 1940s. Her earlier and later stories, are, in general, more lighthearted.
Clueless Mystery: How the clues go together is usually made impossible for the reader to divine until the detective states it.
Although the clues themselves will pretty much always point to the right person. The thing left out is usually details in their motivation.
Connect the Deaths: Frequently used as a Red Herring; at least three novels feature murderers setting up a false pattern to conceal the true motive behind the crime.
Covers Always Lie: Subverted. Christie was greatly displeased with the original artwork for the cover of The Man in the Brown Suit; she felt it was both horribly done and intentionally misleading. From then on, all covers of the original editions required her final approval.
Dawson Casting: In-universe example: in Cat Among the Pigeons, everyone comments on how the supposedly 15-year-old Princess Shaista could pass for 25. Poirot eventually figures out that the real Shaista was kidnapped before she came to England. The woman the kidnappers hired to impersonate Shaista was an actress in her 20s.
Does This Remind You of Anything?: In Hallowe'en Party, Ariadne mentions that she likes apples. One of the teenage boys replies that "It would be more fun if they were melons. They're so juicy. Think of the mess it would make."
The Dog Was the Mastermind: In Crooked House, the murderer turns out to be the ten year old girl who was going around playing detective.
Yahmose, the eldest son in Death Comes as the End.
Percival, the eldest son in A Pocket Full of Rye.
Richard Abernathie, whose funeral is the catalyst for the events in After the Funeral.
Eagleland: As an rather old-fashioned British subject, it should be no surprise that her depictions of Americans in her works were just a little... off. In fact, you pick up early on that if one of her characters is American, Canadian or Australian, or is an British subject who spent any significant time there, that person is either a) the Asshole Victim, b) the killer or c) if not the killer, then a weak criminal type anyways. And of course, there's the case where everyone has ties to America; they all can't have done it, right?
However, being Agatha Christie, that didn't stop her from averting and subverting this trope. In "Sparkling Cyanide", for example, the supposed untrustworthy American character turns out to be a double-agent spy on the side of good, and he also ends up being the one to solve the mystery and prevent the murderer from striking again. The same novel has an example of a British subject masquerading as one of those 'weak criminal type' Americans. He does this in order to slip under the radar of the British detectives - and the radar of any readers too familiar with Christie's other works.
Expy: After noting how much she enjoyed writing the gossipy spinster Caroline Sheppard in The Murder of Roger Ackroyd, Christie decided to revamp this character into someone who would become a fixture in her work — Miss Jane Marple.
Bobby Jones and Frankie Derwent of Why Didn't They Ask Evans? are expies of Tommy and Tuppence. The ITV productions lampshaded this by having both sets of characters played by the same two actors.
Author Avatar Ariadne Oliver's most popular character, Sven Hjerson, is a thinly-disguised expy of Poirot.
Fate Drives Us Together: William Smith and Christina Grant in Murder on the Nile where he continuously courts her and at last asks to marry her. She refuses at first but it is hinted and later revealed that they do get married.
Framing the Guilty Party: In Towards Zero, it becomes apparent quite early on that Nevile Strange is being framed for the murder of Lady Tressilian. However, Strange is actually the killer, and the false clues were planted by himself to make it appear thathis ex-wife was the killer and was trying to frame him.
Alfred Inglethorp of The Mysterious Affair at Styles also does a frame job on himself, albeit for different reasons. Inglethorp sets out a bunch of false clues incriminating himself in the hopes that he will be arrested and tried, at which point he can easily refute the false evidence. Once acquitted, he will then be unable to be tried again due to double jeopardy, even if proof of his guilt turns up later. Poirot foils this plan by refusing to allow Inglethorp's arrest until he has true evidence of his guilt.
Nigel Chapman in Hickory Dickory Dock does what is called "double-bluffing".
Lady Edgeware in Lord Edgeware Dies/Thirteen at Dinnerhires an actress to impersonate her at a dinner party while she kills her husband, then kills the actress later. During the murder she takes great care to be seen by those in the household, though she purposely acts strangely. At first it seems apparent she's the killer, till all the party guests give her an alibi. When the actress's dead body is found, it appears that the killer hired the actress to commit the murder and frame Lady Edgeware. Poirot sees through that though...
Funny Foreigner: Hercule Poirot, so much, but it's also sometimes lampshaded as a deliberately exaggerated act that he puts on to keep people off guard.
Occasionally subverted, such as in the climax of the short story "Problem At Sea", where Poirot demonstrates that he can speak in a flawless Cockney accent if the moment calls for it.
Genre Savvy: Take a shot every time someone compares the current situation to a mystery novel, or even rules out a possibility based on its likelihood in fiction.
Subverted in Towards Zero. It appears that Lady Tressilian was killed with a golf club, but the murder weapon was actually a modified tennis racket. This is important because it means that she was killed not by Audrey Strange, a left-handed golfer, but Nevile Strange, a right-handed tennis player using a backhand.
Greedy Jew: Distressingly, Christie often relied on the pre-War stereotypes of Jews as somehow "other" in her stories and novels, though only occasionally going into truly offensive territory.
She never had a Jew as the criminal (at least, as far as this troper is aware). Rather, she unthinkingly drew on the casually anti-Semitic stereotypes that were accepted in British society of her day. Shortly before the events that led to WWII, she met an actual hardcore anti-Semite and found the experience so disturbing that she went so far as to make a Jew, Sebastian Levine, one of her main sympathetic leads in the 1930 novel "Giant's Bread" (though humorously enough, she still made him a stereotypical Jew— it was just a highly positive portrayal, making this more a subversion than an aversion). She also had the character of Oliver Manders in "Murder in Three Acts", published in 1936, who is another flat-out aversion. Oliver is written as distinctively attractive, though also distinctively "Jewish" and "other", but still ends up as the "good" man in the romantic triangle and gets the girl. ...But then again, "The Hollow" came out in 1946 and has perhaps Christie's worst Jewish character in the form of an irritating boss, the "vitriolic little Jewess" with the "awful corn shrike voice", who was a straight-up portrayal of this trope... It seems that Christie was equally comfortable casting Jews in positive and negative roles. It was just that her construction of Jewish characters (and all non-British characters, really) tended to pull on awkward and dated stereotypes for their assembly.
He Knows Too Much: The surest way for an Agatha Christie character to sign his own death warrant is by attempting to blackmail a killer. Amberiotis of One, Two, Buckle My Shoe and Louise Bourget of Death on the Nile learned this the hard way.
Tommy and Tuppence were called in to identify N or M after an intelligence agent had been killed in a fake accident because he was getting too close.
Also goes for anyone who didn't know that they knew anything significant ( such as Celia from Hickory Dickory Dock, or who didn't know the whole story but knew something vital ( like Miss Johnson from Murder in Mesopotamia). Basically, if you're in an Agatha Christie novel, you'd better hope and pray that you either don't have a major part, don't find anything out, aren't confided to by anyone; or if you do find something out, you know how to keep your mouth shut.
Heroic Wannabe: William Smith in Murder on the Nile (Death on the Nile play counterpart), constantly trying to win the heart of Christina Grant.
Hot Guys Are Bastards : very, very much. The more attractive a man is, the more his appearance departs from the stuffy, military-ish rough and conservative Englishman, the more immoral he is. Attractive men will be suspected immediately, will usually remind the detectives of other handsome killers they've met, and if they're not the killers, they are likely to be accomplices, blackmailers or withholding evidence because they've acquired it while doing something immoral. Plus, with just a few exceptions, any affection of a handsome man for a woman is false, usually motivated by money.
Possibly a case of Write Who You Know. Christie's first husband was a man like this; the marriage ended very badly.
Identical Stranger: In "The Case of the Rich Woman", the title character apparently has a doppelganger who is a poor servant girl. The truth is rather more complicated.
Incriminating Indifference: Subverted in Hickory Dickory Dock. Nigel Chapman, after Patricia is found dead, is incredibly upset, and because of this, the police say that he can't be the killer. Poirot points out that his tears were indeed genuine, and probably more so because he loved her and had to kill her.
Also subverted in the Miss Marple short story "Tape-Measure Murder." The victim's husband doesn't grieve publicly because he believes in the virtue of Stoicism, causing everyone in the village to believe him guilty. Miss Marple is the only one who thinks he's innocent, because he reminds her of an uncle who was also a Stoic.
Used unexpectedly in 'A Problem at Sea': this trope is in play, but not after the murder, but before it. The saintly way the Asshole Victim 's husband put up with her behaviour incriminates him after her murder- everyone wondered how he could be so patient with the awful woman. The truth was, he'd already worked out how to kill her.
Inspector Lestrade: In Hercule Poirot novels, Inspector Japp or Captain Hastings. Inspector Craddock sometimes served this role in the Marple books.
The character of Giraud, in the Poirot book Murder on the Links, is a parody of this type.
Late Arrival Spoiler: Christie's novels occasionally revealed the solutions of previous works, a habit which vexed her publishers. For instance, in Cards on the Table, Poirot makes a reference to the solution to Murder on the Orient Express. The reference is very subtle, but enough to spoil it for someone who has not yet read that novel. Even worse, in Dumb Witness, Poirot casually mentions the names of the guilty parties from four previous novels.
One of the characters in The Pale Horse is one of the suspects from Cards on the Table. If you read the former first, you'll know he can't be the guilty party in the latter. Which wouldn't be such a big deal if it weren't for the fact that Cards on the Table was a subversion of Everyone Is a Suspect where four, and only four, suspects could've committed the murder (and the murderer is one of the four). If you read The Pale Horse first, then you only have three suspects.
Also catches you out if you only watch the ITV film of Cards on the Table and don't read the book, as the relationships end up somewhat different (too long to explain here, but suffice to make this look like Canon Discontinuity.)
Long Runner: The Mousetrap is running continously on the West End since 1952.
Love Hurts: A trope that became a favourite of Agatha's after her divorce from her first husband. When love wasn't driving a character to commit murder, it was guaranteed a broken heart from one side or the other. Only a few pairings survived amidst the body counts and broken hearts.
Love Makes You Evil: Oy. To list all examples would be ludicrous, but Death on the Nile in particular both uses this trope and subverts it.
Market-Based Title: Several of Christie's novels had their titles changed for their US editions. Some of the changes made sense (US readers wouldn't know what a mews was, so the collection Murder in the Mews was titled Dead Man's Mirror after a different story in the collection), while others seem superfluous (Murder on the Orient Express was changed to Murder in the Calais Coach). The editions currently in print have restored the original British titles.
In the case of Murder on the Orient Express, there was a real reason: a mystery novel by the title of Orient Express had come out in the US that same year.
Mary Sue: In-Universe, several short stories have the main characters compare themselves to the iron-jawed supermen/wish they could have the beautiful heroines of the cheap novels they read. At the end, they recognize them for the fantasies that they are, and live happier lives for it.
The Matchmaker: Both Hercule Poirot and Miss Marple display a tendency at times to play matchmaker in the lives of two lovers who need to be brought together during their investigations.
Maybe Magic, Maybe Mundane: The stories involving "the mysterious Mr. Quin", who has a knack for showing up just when something is about to happen, and of saying or doing just the right thing at just the right moment to nudge events in the direction of a happy conclusion. It's heavily implied that he's something other than human, but he never does anything unambiguously supernatural.
Christie was using a symbol that was better known in her time than now, which is why Mr. Quin always shows up just when tragedy in the form of murder or suicide is about to strike—especially when it's about to strike lovers. The harlequin in the commedia dell'arte is both likable and romantic. But in other tales, the harlequin is more sinister—a trickster who makes his own rules and who may bring chaos or death. Quin combines the two images, as he's both a likable romantic and someone who quite enjoys throwing a spanner named Mr. Satterthwaite into the works now and then. And judging by the frequency with which Quin knows when someone is dying or about to die, it's very likely that he's either an agent of death or is Death itself. Christie's words support that; her autobiography, Agatha Christie listed the Harley Quin stories as her favorites and describes Harley Quin as "a friend of lovers and connected with death." Christie dedicated one of the Quin anthologies to "Harlequin the invisible"—which doesn't make much sense if she was talking about a character noted for his sudden and unexplained appearances, but does make sense if she was referring to an entity by a different name.
Mr. Exposition: Mr. Satterthwaite has a talent for describing the backstory lucidly and with the occasional poetic touch, and is frequently called on to exercise it. Played with a bit in that this is also his main detective talent; in the course of describing the situation to another character (and to the audience), he will often notice a detail or correlation of details that points to the solution.
Murder by Mistake: Subverted more often than not. Most of the time, the victim is the person that the murderer intended to kill, even if it appears otherwise. In fact, on multiple occasions, the "intended target" is actually the killer. See Peril at End House and The Mirror Crack'd from Side to Side.
Played straight in Sparkling Cyanide. Victor Drake intended to poison his cousin Iris to gain control of her fortune, but Iris's brother-in-law George ended up drinking from her glass instead of his own.
Murder the Hypotenuse: "Triangle At Rhodes" has two. Marjorie Gold and Tony Chantry want to get married, but they're both already married. They decide to fix this problem by orchestrating a pretend love triangle between Tony, Marjorie's husband Douglas and Tony's wife Valentine, and then killing Valentine, making it look as though Douglas did it while trying to kill Tony, and letting Douglas be hanged for it. Unluckily for them, Poirot sees through it.
Not Allowed to Grow Up: Inverted with Tommy and Tuppence who are the only Christie characters who are shown to age over time from novel to novel that they appear in. They are young lovers when they first appear, but in their last appearance they are shown as a "Grow Old with Me" elderly couple.
Omniscient Morality License: "Are you happy? If not, consult Mr. Parker Pyne" — who will probably lie to you, and on odd occasions may deliberately expose you to physical danger, but you will end up happy. His staff occasionally question his methods, but he's the expert on human nature, so there's not much they can do once he assures them that it's all for the best. (This would be easier to take if there weren't that one story where the twist ending is that Parker Pyne is after all capable of getting it horribly wrong.) It's probably meant to be covered by Rule of Funny.
On the other hand his clients ARE paying for results - and they get exactly what they asked for.
Patchwork Story: The Big Four was originally a series of short stories that were published in The Sketch before being converted into a novel.
Combines with Mushroom Samba: one of the protagonists infiltrating the cult has what she believes to be a genuine religious experience; it turns out later that one of the cult's rituals involves secretly dosing the followers with liquid THC.
Play Name Change: In the play rendition of Death on the Nile known as Murder on the Nile, there are significantly fewer characters than in the book counterpart. Names of some characters have also been changed:
Marie Van Schuyler and Mrs. Allerton merge into the characer Helen ffoliot-ffoulkes.
Miss Bowers and Cornelia Robson merge into the character Christina Grant.
Mr. Fanthrop and Mr. Ferguson merge into the character William Smith.
Hercule Poirot, Colonel Race, and Andrew Pennington merge into the character Canon Ambrose Pennefather.
Simon Doyle's name changes to Simon Mostyn.
Linnet Ridgeway's name changes to Kay Ridgeway.
Jacqueline de Bellefort's name changes to Jacqueline de Severac.
Louise's name isn't changed although her last name is never mentioned.
Put on a Bus: After realizing that Poirot didn't really need a Watson-type character, she quickly married off Captain Hastings and had him move to Argentina (although he periodically returned for more adventures with Poirot).
Recursive Canon: In A Body in the Library, a character listing some mystery novel authors mention Agatha Christie herself.
Red Right Hand: Josephine Leonides' general appearance in Crooked House.
Subverted in Towards Zero. A character tells a story of a child he once knew who he believes got away with a premeditated murder many years ago, who would now be a young adult. He believes this person was cunning, capable of extreme hate and without conscience... but he would know them again, thanks to a certain distinctive physical feature. Unfortunately he happens to die early on without telling anyone what it is and, just so it's not too easy, all the young adults who could be suspected have some unusual scar or quirk of appearance.
Rescue Romance: "The Adventure of the Discontented Soldier" revolves around one being staged to bring two people together, in such a way that neither the rescuer nor the rescuee realises it was staged.
Revised Ending: Christie changed the endings of at least two of her novels when adapting them for the stage.
Shown Their Work: Christie was an expert on poisons and how they worked, and she never missed an opportunity to show this knowledge off.
Sliding Scale of Anthropomorphism: There are a couple of times in the books when the narrative switches to a dog's point of view, with the dogs coming somewhat close to the "Intellectual Animal" point - examples include Bob from Dumb Witness and half a page from Towards Zero.
Also subverted in at least two Poirot stories, where a smashed watch is found at the scene to give a false time for the crime, of course
Strictly Formula: Every Poirot novel has a character at some point say/think 'The man's a mountebank' and is promptly proved wrong. Every Marple novel has someone say/think 'She's gaga' and is promptly proved wrong.
Suicide, Not Murder: Played with in the novels, though Never Suicide ultimately wins out 99% of the time. Occasionally one of a series of deaths will turn out to be a suicide (such as in Dumb Witness), but there is always at least one genuine homicide in each full-length novel. However, in the short stories "The Market Basing Mystery" and "Murder in the Mews", as well as the stage adaptation of Appointment With Death, the alleged suicides do indeed in the end turn out to be suicides.
Tap on the Head: Multiple instances. One notable example is "The Adventure of the Discontented Solider", in which the protagonist is genuinely knocked unconscious as part of a fake abduction being staged for his benefit; it doesn't seem to have occurred to his helpful abductors that he might end up with a unhelpful subdural haematoma (but fortunately, this being Christie, he doesn't).
Theme Naming: Christie was fond of titling her works after nursery rhymes. Novel examples include And Then There Were None; One, Two, Buckle My Shoe; Five Little Pigs; Crooked House; Hickory Dickory Dock; and A Pocket Full of Rye. Short stories include "Sing a Song of Sixpence", "Four and Twenty Blackbirds", "How Does Your Garden Grow", and "Three Blind Mice". Note that each of the first three lines of "Sing a Song of Sixpence" has provided the title of a work.
And the novel Postern of Fate and the short story "The Gate of Baghdad", both of which titles came from the first quarter of the poem Gates of Damascus by James Elroy Flecker.
The Moving Finger, a novel about blackmail in a small English village, gets its title from Edward FitzGerald's translation of "The Rubáiyát of Omar Khayyám": The Moving Finger writes, and having writ/Moves on: nor all thy Piety nor Wit/Shall lure it back to cancel half a Line,/Nor all thy Tears wash out a Word of it.
Unreliable Narrator: The Murder of Roger Ackroyd. Though technically this narrator never misinforms the reader of anything.
The Verse: Although Christie withstood public demands and never put Poirot and Marple in a story together, there was enough crossover among the lesser characters to show that they both lived in the same world. For instance,
A minor character, Mr. Robinson, appeared briefly with each of Poirot and Miss Marple, as well as in Passenger to Frankfurt, which featured neither of them.
He also appears in Postern of Fate, with Tommy and Tuppence.
The Pale Horse, which does not feature Poirot or Miss Marple, does feature Mr. and Mrs. Dane Calthorp (who had appeared in Miss Marple novel The Moving Finger) along with Ariadne Oliver, who appears in several Poirot novels, and Major Despard, one of the suspects from the Poirot novel Cards on the Table.
Speaking of Ariadne Oliver, she first appeared as a supporting character to Parker Pyne, one of Christie's lesser-known detectives; as did another of Poirot's supporting cast, Miss Felicity Lemon.
Superintendent Battle of Scotland Yard, another of Christie's lesser-known detectives (Towards Zero, etc.) is a supporting character in the Hercule Poirot novel Cards on the Table and is mentioned in at least one other Hercule Poirot novel (Hercule Poirot's Christmas); his (by Word of God) son appears in The Clocks, another Poirot novel.
Colonel Race, who also appears as a supporting character in two novels with Hercule Poirot (Death on the Nile and the aforementioned Cards on the Table), but also in The Man in the Brown Suit and Sparkling Cyanide (which are not Hercule Poirot novels).
Another of Christie's lesser-known detectives, the elderly Mr Satterthwaite, collaborates with Poirot in the novel Three Act Tragedy, which ties the supernatural-themed Harley Quin stories into the universe.
The Beresfords have clearly heard of Hercule Poirot enough to make jokes about him and pretend to be him while solving a case in Partners in Crime, and in The Secret Adversary they meet someone borrowing the identity of Poirot's friend Inspector Japp
The Watson: Captain Hastings in the early Poirot novels. After gradually phasing Hastings out, Christie used a variety of one-shot characters in this role. He was brought back for Curtain.
This is sometimes lampshaded. In one story, a girl says something to the effect of, "Dr. Watson, I presume?" when introduced to him and Poirot.
Well-Intentioned Extremist: Poirot. Understanding he has the capacity for this is part of what makes him "help along" his own death in Curtain.
Wham Episode: The Murder of Roger Ackroyd, whose controversial solution not only made Christie a household name, but which completely changed the course of detective fiction. Curtain was also considered this given the events of the story. It was part of the reason Christie locked the novel up for 30 years before allowing it to be published shortly before her death.
Another good example would have to be Jacqueline de Bellefort from Death on the Nile who is the mastermind behind Linnet's murder and murdered two other witnesses who saw too much. And who still manages to be one of the most sympathetic murderers in an Agatha Christie novel.