Agent Scully: Despite being pursued by Satan, meeting a prophet, being seduced by sirens, and being apparently saved from execution by divine intervention, Everett still insists that there is a reasonable explanation for everything. At least it's Lampshaded.
And by the end, he doesn't really seem sure of himself any more after seeing the cow on the roof of a shed, which the prophet told them they would see back at the beginning.
Arson, Murder, and Jaywalking: Some of Homer Stokes' about the heroes near the end of the movie were, "These boys is not white! Hell, they ain't even old-timey."
Black and Gray Morality: Our "heroes" include a trio of escaped criminals and a bad-tempered, corrupt governor. The villains include the Grand Dragon of a KKK chapter and his lackeys, and an Inspector Javertwho may actually be the Devil. The most sympathetic characters are probably Delmar (a sweetly cheerful idiot who nevertheless still goes along with Everett's schemes) and Tommy, who sold his soul to the Devil.
Blatant Lies: "That ain't your daddy. Your daddy was hit by a train."
Book Ends: The film opens with a chain gang together working near a railroad track and singing. The film closes with Everett and Penny's daughters tied together by twine walking over a railroad track and singing.
Also near the beginning they meet the blind prophet on the tracks, who sets up the story; at the end the blind prophet is seen passing by on the tracks.
In-movie also, since the Soggy Bottom Boys' singing is so good it helps resolve the plot.
Brick Joke: The blind prophet at the beginning of the film mentions the trio will see a cow on the roof of a cotton house. Guess what they see after the land is flooded near the end of the film.
There's also a very subtle example that probably went over the head of most viewers. John Goodman's character is clearly modeled on the cyclops of Homer's The Odyssey, with his eye patch and his violent confrontation with the heroes. Goodman's character is later revealed to be a member of the Klan. Though unmentioned in the film, one of the Klan's rankings is "Grand Cyclops."
After mocking Delmar and Pete for being baptized early in the movie, skeptic Everett admits his failings and begs for mercy in a Not So Final Confession at the gallows. He is then forcibly immersed by the floodwaters, and everyone is saved. Literally.
When George Nelson is being taken off to his execution one of the procession for the event yells "cow killer!" See Everything's Better With Cows below.
The Cast Showoff: Real-life blues singer Chris Thomas King plays Tommy, and at one point gets to sing (in his own voice) a rendition of Skip James' "Hard Time Killing Floor Blues." Also, that's really Tim Blake Nelson singing the lead vocal on "In The Jailhouse Now".
Color Wash: They messed with the hue and saturation until everything was an intensely colorful brown, imitating the look of sepia-toned photos.
Contrived Coincidence: Of course the guy the KKK decides to lynch is the one our heroes know and are on friendly terms with.
Corrupt Hick: The insanely corrupt Big Dan Teague. Who is channeling the cyclops Polyphemus.
Cult Soundtrack: The soundtrack album is regarded as one of the most important Country and Bluegrass albums of the decade and sold over 7 million copies. It also won the Grammy Award for Album of the Year in 2002, making it one of only three soundtracks to ever win that award.
Deal with the Devil: Tommy Johnson traded his soul to the devil at the crossroads for his guitar skills.
Deus ex Machina: The flooding happens at exactly the right time to save them all from being hanged. Possibly a literal example.
Did Not Die That Way: inverted, McGill finds out his wife has told his daughters that he got hit by a train, rather than tell them he was sent to jail.
Disney Death: Pete was believed to have transformed into a Toad by the launderer sirens, so they take him in a box. The toad was then killed by Big Dan Teague by being crushed, and his friends were physically incapable of stopping his death because they were beaten to bloody pulps. It was later revealed that the toad was actually not Pete, nor was he even transformed by a toad: Turns out those "launderer sirens" actually delivered him to Sheriff Cooley's men for the reward, and is now a prisoner back at the farm.
Fan Disservice: The Sirens, in addition to being generally beautiful, all wear wet dresses so you can see their lingerie. Yet, combined with the creepy song they keep singing, and the fact that one of them is forcing a drug down Everett's throat, you can't help but feel there's something off about the whole thing.
Funny Background Event: Everett, Delmar, and Pete are all chained together, and try to escape by boarding a moving train. In the foreground we see Everett (on the train) introducing himself to some hobos. In the background, Pete trips before he can climb in...
Also, Pete's gloriously goofy dancing during Delmar's rendition of "In the Jailhouse Now."
Background singing - in Man of Constant Sorrow, Everett finishes singing a depressing stanza that ends in the line "perhaps I'll die upon this train..." and Delmar and Pete chime in with a cheery "Perhaps he'll die upon this train!"
Good Old Fisticuffs: Vernon gives Ulysses a good old-timey ass-whoopin' in the Woolworths. Vernon apparently has some training in the pugilistic arts, whereas Ulysses... not so much.
Go Out with a Smile: George Nelson. We don't see him killed but his last scene is him having been caught by a mob and being led to his execution. He's more then happy with it, however; the mob was even nice enough to give him some violinists as a funeral march.
Hypocritical Humor: Just before he's executed, Everett prays to God to let him see his daughters at least one more time. When the dam breaks and saves him, he starts going on about reason. The other two immediately call him out on it.
Large Ham: Homer Stokes. It's particularly noticeable in the scene where he leads a KKK rally. Of course, it makes sense, given that he's running for governor and a talent for public oratory would help him a lot.
Lyrical Dissonance: The Soggy Bottom Boys' extremely cheerful, upbeat rendition of "Man of Constant Sorrow".
Magic Realism: There are more than a few downright mystical occurrences in the film, such as the prophet, the sirens, and the strong implication that the Warden is Satan.
The way the movie is framed - it starts with a scene of a generic chain gang with no main characters in it, singing as they break rocks, then cuts to black before the actual movie begins - gives rise to the theory that the entire story is being presented as a myth, a subject of chain gang songs, as opposed to "real" events. The pointedly non-realistic bent of many of the movie's events (the KKK marching in a chorus line? And Babyface Nelson being alive three years after his reported death) would seem to indicate this.
Likewise the Governor, whose name is Menalaus, although that's a little more The Iliad.
Musical World Hypotheses: Diegetic all the way through, making its classification as a musical to begin with dubious to some.
With a single pseudo-exception: the three gravediggers' song continues on, through and after the flash flood; of course, if the sheriff really is the devil, the gravediggers could be psychopomps, and not in any danger...
Mythical Motifs: While the film doesn't follow The Odyssey to the letter, it does borrow some notable plot elements from it, such as the Cyclops, the sirens, and one of the main characters trying to get home to his wife so she won't marry someone else.
No Celebrities Were Harmed: There really was a Depression-era Governor named Pappy O'Daniel, but his given name was Wilbert Lee O'Daniel; in the film the governor's real first name is Menelaus (another Homer reference). Also the real O'Daniel was governor of Texas, not Mississippi.
Oh Crap: John Goodman's reaction when he realizes that the fiery cross was coming down directly at him.
Also, Homer Stokes' reaction when he realizes that the town, after his attempt at getting the Soggy Bottom Boys arrested failed, is now going to run him out of town on a rail as revenge for interrupting the performance.
Paper-Thin Disguise: Toward the end of the movie, the fugitive "Soggy Bottom Boys" perform ("Neighborhood of B") while disguised with false beards. Lampshaded later, when their performance wins over the crowd and Everett deliberately yanks his beard off for a moment.
Politically Correct History: Zig-zagged. The white heroes refer to Tommy as a "boy," but otherwise treat him as an equal. The radio station manager insists that he won't play "colored songs," but once the "Soggy Bottom Boys" become popular, Pappy O'Daniel doesn't seem to care that "they's integrated." The KKK is shown in all its silly racist glory, but also portrayed as a fringe organization that is not looked upon favorably by the common townsfolk.
More likely that the townsfolk were more upset by Stokes interrupting the Soggy Bottom Boys' performance by trying to have them arrested and didn't care what else he said.
Well, the Klan had suffered a massive decline in membership in the mid-to-late-1920s due to a series of major political scandals in Indiana, so its not outside the realm of possibility that such affiliation could damage him in early 1930s Mississippi.
Produce Pelting: What the audience does when Homer Stokes ends up interrupting the Soggy Bottom Boys performance to get them arrested, that as well as ride him out of town on a rail.
Real Is Brown: Pursued with a vengeance, given that a substantial portion of the film's post-production budget went into extensive color-correction. The Coens wanted every frame of the film to reflect the dingy, withered dustbowl look, and in some cases took entire fields of green flora and turned them yellow.
Reduced to Ratburgers: Pete and Delmar find a "whole gopher village" and proceed to cook some up. Everett doesn't seem very enticed by the notion of eating rodent.
Retirony: Of a sort. Pete was two weeks from being released from prison anyway. Now that he's escaped, he won't get out until 1987.
Rock Me, Asmodeus!: "And I have it from the highest 'thority, that that negra...sold his soul to theDevil!!!" (the townsfolk don't buy into it, though)
The entire plot contains various shout outs to the Greek poem The Odyssey}} by Homer. The main protagonist is named Ulysses in both stories, has to get home to prevent his wife from marrying someone else and they meet singing women who seduce them (the Sirens) and a one-eyed giant man (the cyclops). The reform candidate is named Homer Stokes, referencing the author Homer. The blind railroad man predicting events references Tiresias, while the blind radio station manager references Homer again, who was also said to be blind.
The KKK scene is based off of the scene in the Wizard of Oz where the Scarecrow, Lion and Tin man try to sneak into the witches castle. The guards are chanting the way the KKK does and even doing a similar dance, and the three heroes steal disguises from the guards/KKK.
The Soggy Bottom Boys are a reference to the Light Crust Doughboys, who were featured on the real-life Pappy O'Daniel's radio show.
There's a coffin floating on a flooded river at the end, which is most certainly a shout out to William Faulkner's As I Lay Dying.
Stern Chase: The Warden's search for the three convicts.
The Stool Pigeon: Pete ends up becoming a Lacerated Larry after the "Sireens" basically turned him over to the sheriff's men for a bounty (which initially led them to believe that Pete was actually turned into a frog due to it being in his clothes).
Surrounded by Idiots - Pappy O'Daniel's cronies and son are sycophantic yes-men who are a bit slow on the uptake, and Pappy is painfully aware of this. This is most likely the reason he tries to convince Vernon T. Waldrip to leave Stokes' campaign and join his.
Working on the Chain Gang: The story begins with Ulysses, Pete, and Delmar escaping from this while chained to each other. Pete, at one point, is recaptured and put back to work on the chain gang and has to be broken out of prison again.