In an interesting cross-media spin-off, Donald Bain wrote several mystery novels inspired by the series, all of which credited Lansbury's character Jessica Fletcher as a co-writer.
This series provides examples of:
10-Minute Retirement: Sheriff Tupper attempts this in one episode, but reclaims his job after his successor Harry Pierce (played by John Astin) proves to be utterly incompetent. The fact that Pierce turns out to be the Killer of the Week didn't help.
Always Murder: Enforced by the show's format, with three notable exceptions:
"Just Another Fish Story": Self-defense.
"To The Last Will I Grapple With Thee": Suicide made to look like murder to implicate someone else.
"The Christmas Mystery": Attempted murder; the victim survives.
Inverted in one episode where the victim's death is thought to be the result of medical malpractice and her doctor is about to be either sued for this or arrested for manslaughter/negligent homicide. Further investigation, however, determines that she was poisoned, meaning it is a murder after all.
"Jessica Behind Bars" subverts this and then plays it straight. The first victim was a Guilt Ridden Accomplice who actually committed suicide, but the second victim was in fact killed because she was a loose thread that may have led to the reveal behind why the first victim took her life.
Amateur Sleuth: Jessica has no formal law enforcement or investigation training, and never acquires any throughout the series; her skills are chiefly based on her career as an English teacher who has a deep understanding of the mystery genre, and applies her understanding of it to Real Life. Once, though, when a witness is unwilling to be questioned with Jessica present, the sheriff says he'll deputize her if necessary.
One murder victim turns out to have been blackmailing the more prominent men of her small town, but then it's implied she was using the money to anonymously support charities for orphans and widows.
The killer in the first episode is being blackmailed over a crime for which he was framed originally, and he kills his blackmailer.
Asshole Victim: The vast majority of the murder victims, with a few exceptions. Rarely is the murderer some brutal and heartless killer; he or she is almost always someone who the victim has wronged in some way, even if the killer themselves turns out to be a jerk. At least one noteworthy exception is an episode about a woman being poisoned to death by her husband, who wants to collect her insurance money and run off with his mistress. At no time is she ever made out to be someone who got what was coming to her, nor is he made out to be sympathetic.
Bluffing the Murderer: Jessica typically finds some way to bluff the murderer into seeking her out, thinking her to be alone and thus defenseless.
Burn the Witch!: The killing of a woman for witchcraft in the 17th century sets up some of the plot in "Fire Burn, Cauldron Bubble," as Seth sees her "ghost." In the present day, there's a Witch Hunter priest who of course is the first suspect when one of the witch's descendants, Irene Terhume, ends up dead. He didn't do it.
Busman's Holiday: Jessica takes a lot of vacations, and death always follows her. It's entirely possible they started having her travel because Cabot Cove would have eventually run out of people.
Cabot Cove's first sheriff is Amos Tupper, an honest cop and a close friend of Jessica. He retires after the fourth season.
Tupper is replaced by Sheriff Mort Metzger, a former NYPD detective who takes the job in the mistaken belief that the town is a peaceful place.
Catch Phrase: Jessica has one just prior to getting neck deep in the current investigation:
Jessica: [to current investigating officer] I know it's none of my business, but... [Insert recommendation of next investigative move].
She also has a variation of her Once per Episode disagreement when the current officer in charge makes the first accusation:
Jessica:*shakes head* I'm not so sure, [officer name]...
Also upon having her Eureka Moment she has a variation of the same basic quote:
Jessica: [Eureka Moment] I believe I know what happened. Excuse me... [cut to catching the culprit and getting a confession]
The Charmer: Michael Hagarty, Jessica's friend in MI6, often invokes this as part of his undercover operations or other crime-fighting activities. Jessica is more often seen to be exasperated by it than anything, since he frequently uses it on her to hide his true motives for doing something.
Chekhov's Gun: Inevitably, at some point in an episode, the camera focuses just a little longer than necessary on something utterly random and mundane that turns out to be significant regarding the identity of the killer. Occasionally, this happens even before the actual murder does.
Chuck Cunningham Syndrome: Ethan Cragg (Claude Akins), who appears often enough in the first season to be considered a recurring character, disappears without explanation at the start of season two. His role as Cabot Cove's resident lovable curmudgeon is filled by Dr. Seth Hazlitt for the rest of the series.
Clear My Name: Naturally, this happens in a lot of episodes, where an innocent man is the prime suspect of the murder (although it's usually not an intentional Frame-Up on the real killer's part) making Jessica's task twice as hard.
Contrived Coincidence: One episode revolved around Jessica getting sued when the plot of her latest novel is staggeringly similar to a real life murder case. Jessica is shocked when she reads the reports from the actual case and realizes it is similar, even though she's never heard of it. In fact, the book is so similar that the guy who is suing her uses it to discover who actually murdered his wife and subsequently gets killed by the same culprit: his former secretary/second wife.
Creator Breakdown: Reversed, in-universe. The reason Jessica started writing her mysteries in the first place was to give herself an outlet to work through her grief over her husband's death. In a later episode, Seth expresses concern that she's investing too much of her time in her books and missing out on life.
Crossover: With Magnum, P.I.. The crossover episode is a two-parter, with the first part on Magnum, P.I. and the second part on the subsequent episode of Murder, She Wrote. Luckily, the appropriate crossover episodes are included on the corresponding season DVDs.
Dirty Cop: A few episodes have them as the culprit.
Doom Magnet: Is Jessica Fletcher visiting your town or home? One of three things is going to happen. You'll either be killed, accused of killing, or be revealed as a killer. Being accused is your best bet, as Jessica will invariably prove you innocent when she reveals the real killer.
Lampshaded more than once.
Dude Magnet: Jessica gets a lot of attention from older gentlemen, and in one episode a college-age Cake Eater flat-out propositions her.
Eagle Eye Detection: Jessica's ability to pay attention to minute details is usually what reveals the killer. In one example, Jessica notices that one character has been hiding his hand in his pocket since the murder because a dog bit him after he committed it.
Eureka Moment: Once per Episode with some minor detail that ties all the other evidence together, and leads into the confession scene. Seth and Mort lampshade it in later seasons with exasperation as they see her staring off into space, realizing she's figured it all out. Subverted in one episode when she does this... and then announces she's worked out the ending to her latest book.
Everybody Laughs Ending: The show rolls the end credits on Jessica's laugh in roughly 99% of the episodes. The few which don't end with her laughing (such as the pilot) end instead with a freeze shot of her astonished face as she realizes something important or is taken by surprise in some way.
Fictional Document: Jessica's novels; several of the titles are named in the series, such as her first book, The Corpse Danced at Midnight. Some of the actual crimes are also solved with these providing key evidence.
Friend on the Force: Jessica acquires a large number of friendly detectives as the seasons go on, not to mention her friendships with the Cabot Cove sheriffs.
Fully Absorbed Finale: The "bookend episode" The Grand Old Lady was actually an unfilmed series finale script from a previously-produced and cut short Ellery Queen series, albeit with the characters slightly changed to dance around rights issues. Both series were produced by Levinson and Link, and they apparently decided not to let a good script go to waste.
He Who Must Not Be Seen/Dead to Begin With: Frank Fletcher, Jessica's late husband, who passed away prior to the series. Throughout the series, Jessica interacts with her multiple nieces and nephews, some which belong to Frank's side of the family. At one point, she even reunites with her brother-in-law and other relatives connected to the family. The only thing the viewers have as far as a description on Frank is that he and his brother looked alike and they brought in an actor to provide his voice.
Identical Cousin: Jessica has a British cousin named Emma, also played by Angela Lansbury. In the episode "Runs in the Family", only Emma was featured, with Jessica not appearing at all, and Emma proved to be a rather good sleuth herself.
Instrument of Murder: "Death to a Jazz Beat" offered an inversion; a jazz clarinetist, who was planning on firing his back-up band before signing a huge recording contract was murdered by a poisoned clarinet reed. In another episode, a ballerina was murdered by touching a poisoned prop.
Just Between You and Me: Some of the not-so-nice killers always confess to Jessica or someone else in the belief that they are alone, just before trying to kill them.
Law of Inverse Fertility: Jessica and her husband Frank were childless; in a first-season episode, when speaking to a new acquaintance (who later turns out to be the murder victim), she explains that they "were never blessed that way," suggesting that they wanted children but couldn't have them for whatever reason.
Love Makes You Crazy: Many of the murders are done for loved ones, but there are a few where the affections made the culprit quite loopy.
Magazine Decay: In-Universe example: a publishing magnate buys up a literary magazine called Literary Lines and adds Maxim-esque pictorals of bikini-clad ladies. This doesn't sit well with Jessica, who is under contract to have her first short story published in the magazine.
It's been calculated that Cabot Cove has a murder rate of 86 per 1000; by comparison, the most murderous city in the world, Caracas, has a murder rate of 1.1 per 1000. That's not counting the murders that happen outside Cabot Cove...
In some years, more people were murdered in Cabot Cove on the show than were murdered in the entire state of Maine in Real Life.
Lampshaded in one episode when another character tells Jessica, "If murder were a disease, you'd be contagious."
Lampshaded again by Sheriff Metzger, a former New York cop who, after a year as the sheriff of Cabot Cove, asks Jessica, "Just what the hell's wrong with this town?"
Nephewism: Jessica had no children, but many nieces and nephews. Grady Fletcher was the main repeater among them. As noted above, it's explained in a few episodes that Grady was orphaned as a little boy and was raised by Jessica and Frank; as their surrogate son, he had good reason to be a repeater.
Jessica's other repeating relatives were a niece and nephew-in-law, played by Genie Francis and Jeff Conaway, who had a continuing thread about him wanting to be in the entertainment business.
Obfuscating Stupidity: Jessica doesn't hesitate to play up her "little old lady" image to get information out of people. In a few instances she even plays a drunken floozy with surprising skill, which may be justified since Jessica has a history of theater in her background.
Oireland: "The Celtic Riddle", "Nan's Ghost", "A Killing in Cork", "Another Killing in Cork", "To the Last Will I Grapple With Thee" (set in New York but involving an Irish blood feud)...
Pac Man Fever: Surprisingly averted for a show focused on murders and generally populated by adults who are focused on anything but video games (save one episode where the murder occurred at a virtual reality game developer). In one early episode, Jessica's friend Ethan is playing a Spy Hunter arcade game. When footage of the game is shown it is perfectly accurate, complete with the player's hands making movements and the vehicle on screen matching them.
Platonic Life Partners: Jessica and Seth have this dynamic. They obviously care about each other, but there's little to no indication of anything romantic between them.
Police Are Useless: Often, but not always. Generally, if a police officer/federal agent/other law official is willing to listen to Jessica, they're portrayed as sensible and helpful. The useless ones are those who tell her to stay out of their way.
Poorly Disguised Pilot: In everything but intent. When Angela Lansbury started to tire of the pace of a weekly network show, a strategy was devised that would allow the network to do a full season without Lansbury having to do a full season. Slightly more than half of the episodes of the season would be full adventures of Jessica Fletcher. The remainder would be Poorly Disguised Pilots, for which Lansbury, as Fletcher, would film bookend sequences, explaining the new character we'd be seeing for the next hour. Sometimes these were Jessica's own fictional characters, but other times they were friends or relations of Jessica's, such as Dennis Stanton. They weren't really intended to spin-off any of the characters - although if any were exceptionally successful, why not?
Put on a Bus: The original sheriff retired when Tom Bosley left the show.
Red Herring: Usually Once per Episode there will be someone who seems all too obviously the killer. They hated the victim, they are likely angry individuals, they have a shady record, they certainly have means and motive, they made some kind of threat to the victim, and/or they're likely the first one implicated by evidence (which incidentally is almost a guarantee for Jessica to prove their innocence, or have them end up murdered by the true culprit).
Played with once with Kate Mulgrew in her third appearance. The killer actually speaks during the murders and sounds like a disguised version of Kate's distinct breathy voice. Kate played the killer in two previous episodes, so it seemed likely that she'd be it again.
Retool: The eighth season began with Jessica moving to New York City to take up a teaching position. J. Michael Straczynski (of Babylon 5 fame) had taken over as head writer for the series alongside a new producer so the decision was made to move away from Cabot Cove (which would also allow for more varied storylines and less Cabot Cove Death Syndrome, which was starting to get a little absurd). Episodes set in Cabot Cove were still produced but not to the same extent as the first seven seasons.
Scoundrel Code: Dennis Stanton was a Gentleman Thief who, after going straight, became a recurring character. During his burgling years, he maintained his own strict code of conduct: never steal anything his victims couldn't afford to lose, never steal anything of sentimental value to the victim, and only steal items insured by a specific insurance company. The last one is for personal revenge, as the company in question refused to pay for a treatment that could have saved his wife's life.
Script Swap: Done in one episode with an aging actor whose memory is so bad he has to rely on the teleprompter. While this looks like an Engineered Public Confession, it is actually a ploy on Jessica's part to trick the real killer into exposing themselves.
Sherlock Holmes: In the pilot, a man dressed as the Great Detective is the murder victim.
Swiss-Army Weapon: Dennis Stanton's umbrella. It has, among other things, a dart launcher in the tip, and the handle seems to have both a blade attachment and a lock pick. That's not including Stanton's own expertise with it as a weapon on its own.
Sympathetic Murderer: Frequently killing the Asshole Victim. Sometimes it's self-defense or accidental; in one case of the latter, Jessica agrees to keep quiet about solving the murder because revealing the "killers" would cause harm to an innocent person. However, some of the murderers lose their sympathy when they try to frame someone else or try to kill Jessica to cover it up.
One of the show's final episodes, "Murder Among Friends," features a murder taking place among the cast and crew of a show, titled Buds, about six young people trying to make it in the big city. Sound familiar? The show got slotted against Friends as part of a plan to kill it off.
The final episode, "Death by Demographics", similarly knocks on the reason for the show being death-slotted: its lack of appeal to the 18-39 demographic. The episode itself is about a radio station manager who fires every employee over a certain age and changes the station's classical music format to more contemporary hard rock.
Technology Marches On: During the run of the show, Jessica goes from typing manuscripts on a typewriter to using a computer, as reflected in updated opening credits. The final movie shows that Jessica has even begun using a laptop.
Trans Atlantic Equivalent: More than a few people have mentioned that Jessica Fletcher could be considered an American Miss Marple. This is especially hilarious because Angela Lansbury has played both. In fact, the opening of "The Murder of Sherlock Holmes" is practically a direct lift from the opening of The Mirror Crack'd, the film in which Lansbury played Miss Marple.
This even happens with recurring guest stars. Ron Masak, before joining the cast as Sheriff Mort Metzger, appeared in two earlier episodes (the first playing a cop in New York!).
William Windom played one of the guilty party in "Funeral at Fifty-Mile" before appearing as Dr. Seth Hazlitt in the second season.
Madlyn Rhue, before playing Cabot Cove's librarian in later seasons, appeared earlier as a victim's widow in "Seal of the Confessional".
Another interesting example is "Murder on Madison Avenue": Firstly, after having played Jonathan Quayle Higgins in the aforementioned Cross Over with Magnum, P.I., John Hillerman appears in this episode as a completely different character, to the likely confusion of longtime viewers. Secondly, Barbara Babcock's character (her fourth on MSW, fifth if you count The Law and Harry McGraw) is murdered by her assistant (played by Hallie Foote), but in an interesting possible Casting Gag, the next season episode "For Whom the Ball Tolls" sees Babcock and Foote together again (playing completely different characters, of course), working together on a historic preservation committee, as if nothing had ever happened.
Leslie Nielsen appears in the first season as the captain of a cruise ship when Jessica gets involved with a murder aboard the ship. He turns up again in season three as an old admirer of Jessica's who spots her on the street, plants a huge kiss on her lips, then shortly after gets accused of murder.