John Nettles as Inspector Barnaby and Jason Hughes as DS Jones.
Eastwood: ...Barnaby is about to have the bolt of inspiration that cracks the case, solves the murders and the artifact-smuggling ring, and find out that the postmistress is getting off with the vicar, who's secretly her uncle.
Lothar: How many times have you seen this episode?
Actor Allusion : In Vixen's Run, the plot kicks off when elderly Sir Freddy Butler dies at dinner. Despite there being traces of strychnine in his system, the coroner rules natural causes as strychnine is an ingredient in heart medication and Sir Freddy was rather old, fat and drunk. Several other members of his family are killed over the course of the episode by Sir Freddy's first wife Lady Annabel, played by Sian Philips. When she's taken away by Barnaby in the police car at the end she says
Lady Annabel: I suppose you want to know how I killed Freddy.
Barnaby: But Sir Freddy died from natural causes.
Lady Annabel: Oh... of course.
Now remember what Livia did to her husband (and much of the rest of the cast) in I, Claudius...
Aristocrats Are Evil: Most of the nobility/very old established families are complete assholes , more obsessed with their lineage and money than murders.
Attack of the Town Festival : "The Straw Woman" had a village deciding to go ahead with a festival despite the vicar being burnt to death. The replacement vicar was then also murdered.
Awful Wedded Life: Nearly half the couples seen. In one case, a woman was not only a witness to her husband's bizarre murder (see Grievous Bottley Harm), she calls out corrections when the murderer misses.
Bad Habits: Jones disguises himself as a nun to trap a muderer in "A Sacred Trust".
Subverted in "Shot at Dawn", where the prospect is raised with respect to a newly-engaged couple who are unaware that her mother and his father have been carrying on for decades; the mother only laughs and says that she's been very careful to ensure none of her children are the result of the affair.
Forms part of the backstory to the murders in "Dark Secrets". Once Barnaby realises this, the murderer becomes obvious to him.
Butt Monkey : Poor Jones. Will anyone EVER treat him nicely? (Troy and Scott also received this treatment, but had a tendency to bring it on themselves.)
Chuck Cunningham Syndrome: The show never actually explains what happened to Scott. His absence in the episode that introduces his replacement Jones is explained (he called in sick — Jones was a temporary replacement), but by the next episode Jones is there to stay without Scott being mentioned again.
Couldn't Find a Lighter: In "Ghosts of Christmas Past", Jerkass Digby lights a cigar from one the candles on the dinner table as a taunt at one of the other guests who is not allowed to smoke.
Cowboy Bebop at His Computer : Midsomer is a county, not one village, contrary to what idiot TV announcers and others will tell you. The fact that there's an actual English village called Midsomer Norton really doesn't help.
Although Badger's Drift is a very common locale where (judging by rough guess) everyone in the town has died or been a murderer. Including the priest. It's also one of the few non-Midsomer titled locales and is close to many of the others (you see signs pointing to Badger's Drift often). So it does fit the "deadliest village in Britain" tag.
In Australia it's "deadliest county" in the ads, which are often edited to highlight the more tart dialogue, thus making them quite amusing.
Detective Mole: Sgt. Trevor Gibson in "Sleeper Under the Hill" turns out to be involved in the killings and does his best to throw Barnaby and Jones off the trail. He ultimately falls victim to his partner in crime.
Dirty Cop: Sgt. Trevor Gibson in "Sleeper Under the Hill".
Barnaby: Troy! ... You were driving on the wrong side of the road.
Dry Crusader: A sect of these appear in "The Night of the Stag".
The Eeyore: The Reverand Giles Shawcross in "The Sword of Guillaume".
Enfant Terrible: At least two episodes have had children as the murderer (though one just masterminded the whole thing using his mentally-retarded uncle to do the killing).
Enhance Button: Usually Averted due to the show's rather classic detective approach. There's one in "Days of Misrule", though.
Entitled Bastard: A common trait among the gentry and "old families" of Midsomer County. In one episode, one such gentleman brushes off accusations of conspiracy to commit murder by saying that, as a scion of England's old wealthy families, "we make our own rules." (And he has nothing on the episode's murderer.)
Even Evil Has Standards: One murderer takes a small boy who he knows to be the witness to his crime out on a boat ride. Then Barnaby has his Eureka Moment and heads for the dock, expecting the worst... only to find the kid unharmed, with the murderer saying he couldn't kill him.
Geographic Flexibility : The villages often gain features and places previously unseen or unheard of. The series is filmed in locations all around England and Wales. It shows. But, surprisingly, it mostly averts California Doubling. The use of this trope is to be expected, given how the series is one of the Long Runners of British TV and is set in a small fictional English administrative region with a predominantly rural, old-timey character.
Girl on Girl Is Hot : Actually averted with Troy, who's a bit of a homophobe and never considers lesbians as arousing.
Gory Discretion Shot: Although the series has never shied away from depicting some spectacularly nasty murders, viewers are generally spared the worst of it. "The Sword of Guillaume", however, averts this trope to the point of horror.
Happily Married: Tom Barnaby and his wife Joyce, apparently they are the only happily married couple in Midsomer.
There are a few other Happily Married couples, they're often the murderers.
Headless Horseman: In "The Dark Rider", a killer lures several victims to their deaths by masquerading as a headless horseman from local legend.
His Name Is...: Classic case in "The Glitch". The mechanic leaves a message for a friend indicating he knows who the murderer is, but he's murdered before they can talk. He leaves a cryptic clue at least.
Jack the Ripoff: In the episode "Echoes of the Dead", Barnaby recognises the murders as recreations of famous murders of the early 20th century, except for the last one, which he's not able to place. When the murderer is caught, Barnaby asks about it, and the murderer shrugs and says, "I was in a hurry and I couldn't think of anything".
Madwoman in the Attic: As alluded to in the title "Left For Dead", a kid who was seemingly killed 19 years prior turns out to be alive, brain damaged, and locked in a cellar convinced by a couple that he was their lost child. When he finally figures out his true identity, he goes on a Roaring Rampage of Revenge.
The Mistress : As likely as not, a mystery might involve this or adultery.
Monochrome Casting : The producer, Brian Tru-May, got fired for telling the Radio Times that they didn't cast non-white actors, or have non-white characters, because they wanted to remain "the last bastion of Englishness". Indeed. Apparently "the last bastion of Englishness" involves a murder rate higher than The Wire.
Sgt Scott: "Sir, I just got here, and we already have three bodies." DCI Barnaby: "It has been remarked upon before, yes."
New Media Are Evil: "Picture of Innocence". The plot revolves around Digital vs Traditional Photography.
New Neighbours as the Plot Demands: Many an episode features characters whom Barnaby has known for years, but whom the audience has never seen before and for the most part will never see again.
No Badge? No Problem!: One episode has Barnaby be removed from a case because his wife is tangentially connected to it. His replacement being a perfectly intolerable little dipstick, Barnaby gets to the witnesses first without mentioning he's not on the case.
Not In My Back Yard: The title (and main theme) of an episode. Unpopular development plans often end in murder in Midsomer.
Orgy of Evidence: In "Fit for Murder", Barnaby and Jones find a large amount of incriminating evidence when they search the house and vehicle of a pair of suspects. Barnaby points out the murders were methodical and carefully premeditated, and scarcely the work of someone who leave incriminating evidence (that they had no reason to keep) where any search would reveal it.
The Password Is Always Swordfish : In "Market for Murder", the password on the Reading Group's secret share market account is 'Gerald'; the name of the late husband of the group's founder (whom she could not go five minutes without mentioning in conversation). Somewhat more acceptable than normal, given that she was borderline obsessed with his memory and probably could not help herself in using his name as her password.
Pick a Card : In the episode "Ghosts of Christmas Past", a boy who wants to be a magician when he grows up does an actually-quite-clever version of the trick while being interviewed by Barnaby and Scott about the murder, and his explanation of how he did it (including the fact that he arranged matters to have his own choice of card come up at the end) inspires Barnaby's later Eureka Moment.
Self-Referential Humor: At one moment, the brass band from the episode "Things That Go Bump in the Night" plays the series' main theme.
Serious Business: Some of the murders have unbelievably ridiculous motives (to anyone but the murderers). One woman ends up killing three people because her driving drunk would bar her from joining the village social club for life.
Shared Family Quirks: One episode has Barnaby figure out two women are related when both use the same bizarre expression despite not living near each other, just in time to save the Asshole Victim.
Shout-Out: To the famous album cover of The Beatles' Abbey Road. An artist/forger hides errors in his forgeries as a joke. One landscape painting, which he claimed to be centuries old, includes four men in the distance who on close inspection are clearly John, Paul, George, and Ringo. ("The Black Book")
Slut Shaming: "A Sacred Trust" involves some romantic liaisons, including one girl shamed for her involvement with a jock.
Smite Me, O Mighty Smiter: The gloomy vicar in "The Sword of Guillaume". Naturally (for this programme), he's given a sign just at that moment.
Stress Vomit: One character immediately starts vomiting when she learns the guy she's been banging through the whole episode is actually her nephew. He doesn't really react to that, given that he also just learned he's the product of Brother-Sister Incest.
Surprise Incest: This was in an episode where a man had 'spread his seed far and wide' — you could hardly turn a corner without finding one of his bastards. One couple didn't meet until they were both in graduate school in Canada and got married, only later realizing they were half-siblings; she was totally squicked, he didn't mind.
A character in "Dark Secrets" immediately vomits upon learning the employee she'd been having an affair with was her nephew, the nephew himself being the product of Brother-Sister Incest.
Suspiciously Similar Substitute : When John Nettles left the series, DCI Tom Barnaby retired, to be replaced by the younger and more energetic DCI John Barnaby. Apparently he's a cousin, and they did hang a lampshade on it in the episode where John Barnaby was introduced, but really, it's like they're not even trying. The name at least might be chalked up as an Enforced Trope due to the show being called "Inspector Barnaby" in certain other markets (France, Italy, Germany, and Japan according to The Other Wiki).
Tag-Along Actor: Cully's actor boyfriend rides along with Barnaby and Jones to research the role of a detective sergeant. It's a comment of his that gives Barnaby the Eureka Moment.
That One Case: George Meakham's obsession with the original Strangler's Wood murders.
One episode has two primary school kids sneaking vodka and cigarettes.
This was a major plot revelation in "Left for Dead" (series 11). A group of four kids allow another kid to tag along while they smoke and drink. When that kid gets uncomfortable, they torture him, drown him in a river, and toss his body down a well.
Trust Building Blunder: DCI Tom Barnaby's (predictable) contempt towards team building exercises is on display in "Days of Misrule" when he is forced to go on one by the new chief superintendent. Hilarity Ensues.
Vehicular Sabotage: "Death in the Slow Lane". The episode "Death and the Divas" from season 15 also had a death involving this.
The Vicar: Almost a prerequisite for any whodunit set in an English village, though dog collars appear to be the Midsomer equivalent of a Red Shirt. If you're a clergyman in Midsomer, chances are you'll either be horribly murdered or unmasked as a horrible murderer before the credits roll. Of particular note is the Reverend Stephen Wentworth, played brilliantly by Richard Briers in the episode "Death's Shadow". Another honourable mention should go to Mark Gatiss's Giles Shawcross in "The Sword of Guillaume".
What a Drag: One victim in "Blood on the Saddle" is killed by being lassoed and dragged along behind a horse.
With This Ring: In "Schooled In Murder", John Barnaby buys a ring for his wife for their 15th anniversary. However, while distracted by Jones, he accidentally feeds the ring to his dog Sykes. He then has to fabricate reasons to keep the dog with him till he can, um, retrieve it.
Woman Scorned: Quite a few cases. Patricia Blackshaw in "The Black Book", for one.
Yandere: quite a few of the murderers in the 1st series.