In the comics, both Superman and his adopted father Jonathan Kent are very idealistic and optimistic people, hoping for the best in people. In this universe, Jonathan was pretty paranoid because of Clark's origins as an alien and felt The World Is Not Ready for Clark. As a result of this and constantly being a Hero with Bad Publicity despite his efforts, this version of Clark is also more reserved and melancholic, suffering feelings of alienation, sensory overload and survivor's guilt. Superman's personality is significantly realigned in both versions of Justice League in order to bring him more in line with his classic portrayal, as part of the intended arc to have him grow into the role.
His lieutenant Faora is also changed; in the comics she's a violent misandrist and a serial killer who teamed with Zod mostly for benefit. In the film she's a Noble Demon, his loyal dragon, and isn't shown to have any particular hatred for males. (This is part of the confusing history between her and Ursa, Zod's usual Dark Mistress; they're Decomposite Characters whom this movie combined back together.)
Lex Luthor is very different than the comic version, being portrayed as a sort of Psychopathic Manchild; apparently the idea was to modernize him as an eccentric young tech billionaire, but most fans find him much less threatening than the usually calm, Stoic that he is in most modern adaptations.
Much like in Tim Burton's film (and Chris Nolan's reimagined series, however much the titluar character says otherwise), Batman is more open to the idea of lethal force, albeit against mercenaries rather than mentally ill people. This version for a time also branded rapists and human traffickers for other criminals to target. However, he begins to undergo Character Development that sees him backing off these more extreme methods and becoming closer to his comic counterpart. Superman's resurrection via the Genesis Chamber and a Mother Box in JusticeLeague is something Bruce decided, as opposed to his traditional opposition to the use of the Lazarus Pit in the comics, though it's also implied that it was also motivated by guilt over his actions against Superman in BvS.
In the comics Barry Allen/The Flash is an enthusiastic-yet-humorless dork. Here he's still a dork, but he also has the Deadpan Snarker trait that's typically associated with his successor Wally West.
Cyborg is far more stoic than most portrayals of him, the Re-Cut made him more lighthearted similar to his Teen Titans personality. The Snyder Cut clarifies that this was part of his character arc and it's implied in the epilogue that he would be much more upbeat if he ever made a future appearance.
Steppenwolf in the comics (and the theatrical cut of Justice League) is little more than a Blood Knight who is audacious enough to tell Darkseid he doesn't think that highly of him despite being one of his trusted enforcers. In the Snyder Cut it's mentioned that he was initally part of a coup against Darkseid and has long since repented for his crimes. He also doesn't seem to get much joy out of conquering planets unless he's fighting someone like Wonder Woman.
Amanda Waller while just as much a General Ripper as she in the comics, would never as go far as shooting her employees in cold blood like she does in film. In fact the preservation of innocent life, tends to be her number one priority in the comics.
The Joker is better off mentally and morally in the film. In the comics he’s a Chaotic EvilMonster Clown who abuses Harley, in the film, he’s just a somewhat Ax-Crazy mafia boss who does genuinely care for Harley and even rescues her. Birds of Prey (2020) corrects this decidedly stating Joker is a vicious boyfriend to Harley who soon leaves him, but in the comics Joker is known to stalk obsess over Harley even if she cuts ties with him, while he apparently just gives up on her in the film. Zack Snyder's Justice Leagueflips it again as during the Bad Future segment, Joker while still unpleasant is shown to be a Retired Monster and The Atoner who has a shaky truce with Batman, which is actually genuine. In the comics and most media the Joker usually goes back to trying to kill/hurt Batman after the Enemy Mine ends.
Deadshot is an unabashed nihilistic Psycho for Hire who regularly kills innocent life for a decent pay check in the comics. In the film, his Pet the Dog relationship with his daughter Zoey is given greater emphasis than his mercenary life. Deadshot is also quite stoical in the comics while he’s jokey and full of one liners on the screen.
Harley Quinn while she’s steadily become more heroic in recent years is still a unstable Psychopathic Womanchild who too eagerly dispenses Disproportionate Retribution to anyone she takes issue with. In Suicide Squad, Birds of Prey and The Suicide Squad she’s mainly just childish and only becomes violent in self-defence or to protect people she likes. She also has leadership qualities in her second film outing, which she happily lacks in the comics.
Captain Boomerang isn’t a racist, misogynistic and homophobe who actively provokes people like in the comics. The only unsavoury traits he has is drunkenness and a fetish for unicorns.
Rick Flagg is a bold and occasionally Hot-Blooded soldier in the comics, in the film is purely The Stoic and is also willingly to trigger the Explosive Leash on Slipknot and turn a blind eye to Waller’s tyranny, unlike the comics where he’s the most moral member of the organisation. Though he’s got a softer side especially when concerning his girlfriend June Moon.
Fausta Grables goes from a dangerous Swiss WWII era Nazi agent in the comics to a (seemingly German) socialite who's invited at General Ludendorff's gala during World War I in the film.
Diana herself is far more naive than she was first presented as in the Golden Age or most modern depictions, being a complete Fish out of Water. She’s also a Composite Character of her previous and current portrayals, like early continuity she’s compassionate and idealistic, but she’s also more than willingly to use lethal force like she is in moderncontinuity.
Mera is a prominent Fiery Redhead in the comics whose temper allowed her to join the Red Lanterns, in Justice League and Aquaman she’s very calm even in the face of her Love Interest Arthur’s idiocy and recklessness. She does get angry at him on few occasions but it’s not to the Hair-Trigger Temper levels of her comic version who’s quite a clingy wife at times.
Orm is more a Tragic Villain whose motivations and anger towards Arthur are due their mother Atlanna being banished by his father due giving birth to Arthur out of marriage. In comics he is a racist tyrant who even when working with the heroes, still tries to seize the throne at first opportunity. Also in the comics Orm hates Arthur while he has some level of Villain Respect and brotherly care for him in the movie.
Vulko is a scheming bastard in modern comics who wants vengeance on Orm and manipulates Arthur in order to take control of Alantis and invade the surface world. In the film he’s a benevolent Mentor Archetype to Arthur, although Vulko’s classic characterisation was heroic as well.
Doctor Sivana is an unrepentant Mad Scientist in the comics, in film he’s an incredibly sympathetic Tragic Villain who has suffered great abuse from his father and brother and corrupted by the Seven Deadly Sins.
Mary in most continuities is either a Plucky Girl or just upbeat, in the film she’s more mature, quiet and is depressed at the prospect of leaving her adoptive family.
Freddy like his adoptive brother has more of a personality than his classic counterpart but unlike his New 52 portrayal isn’t a lying con artist.
Cassandra Cain is a solemn and highly trained Tyke Bomb who’s generally compassionate and righteous when not beating the snot out of criminals. The movie version is a Bratty Half-Pint street kid who’s quite sarcastic, she’s also not mute like in most comics.
Black Canary is a jaded and submissive Broken Bird in the film who initially works for Sionis before defecting. In the comics Dinah is a passionate crimefighter who remained steadfast even after her mother’s death and would never work for someone like Black Mask.
Huntress in the film has anger management issues and displays reluctance to take on her role as a vigilante. In the comics while she is far more ruthless than Batman, she’s still usually quite calm and full of confidence.
Renee Montoya talks and acts like a hardboiled 80s cop, which differs greatly from her animated and comic version where she’s a righteous cop and sometimes aloof.
Roman Sionis aka Black Mask while he’s just as loathsome and violent towards women as he is in the comics is nevertheless far more reasonable being quite the Benevolent Boss to his (male) underlings. In the comics he’s just a pure evil nutcase with no redeeming traits, relishes pain and will happily kill his Mooks if they get in his way or fail him.
Zsasz in the film is a incredibly loyal and illiterate henchmen of Sionis who shows a bit of of his sadism now and then. Comic Zsasz is, far, far worse being a horrific killer even by the standards of Batman’s Rogues Gallery. Unlike the film, comic Zsasz is very intelligent albeit maliciously so.
Bloodsport is a largely two dimensional Big Scary Black Man and Psycho for Hire in the comics. In the film he’s an affable Punch-Clock Villain turned Anti-Hero who cares for his daughter as well as daughter surrogate Ratcatcher II and even decides to be a hero by the end of the movie. This is partly due to making Bloodsport a Composite Character of Deadshot, right down to the estranged daughter.
Rick Flag is Truer to the Text compared to the 2016 film, being a Father to His Men and pretty much The Paragon of the team. He’s also unwilling to be a lapdog to his country, as upon discovering America’s involvement in Project Starfish, he betrays Waller and wants to make sure the world knows what atrocities took place.
Peacemaker while still a Knight Templar is much more humorous and even crude in his attitude compared to the serious Ax-Crazy vigilante with Black-and-White Insanity from the comics. He’s not as much The Unfettered as when forced to fight and kill Rick Flag and then attempt to kill Ratcatcher II as well under Waller’s orders to keep Project Starfish secret, he shows genuine regret and hesitation especially with the latter, unlike his comic counterpart who kills without a second thought.
Harley Quinn compared to the 2016 film, is much moral and loyal to the team. As seen when she shoots Silvio Luna when he talks about murdering women and children, in contrast to her first film where she was an accomplice to Joker in the death of Robin. Although this likely due to her better qualities and character arc from Birds of Prey being carried over into this film.
Ratcatcher in the comics is a skulking creep who values rats over human life. In film both Ratcatcher and his daughter/successor Ratcatcher II are Not Evil, Just Misunderstood and very humane, using rats to help others rather than just to plague Gotham.
Polka-Dot man is a IdiosyncrazySmug Snake and Boisterous Weakling in the comics. In the film he’s very much The Woobie, being deeply psychologically scarred thanks to his abusive Mad Scientist mother. He’s even capable of outright heroism unlike his often pathetic comic version.
Nanaue aka King Shark is while bloodthirsty and savage is still intelligent in the comics able to speak and plot. In the film, he’s mainly got Hulk Speak although compared to the comics he’s more of a Bruiser with a Soft Center, especially around Ratcatcher II.
Both Blackguard and Savant are Super Zeroes with none of the noble qualities of their comic selves. Blackguard being a traitor and Savant a Dirty Coward.
Although he only appears in one scene, Calendar Man is portrayed as a bullying Jerkass unlike the soft-spoken Affably Evil character from the comics.
Weasel is a intelligent Beast Man in the comics, whilst his film counterpart is a mindless drooling beast.
Chaotic Evil Monster Clown who abuses Harley, in the film, he’s just a somewhat Ax-Crazy mafia boss who does genuinely care for Harley and even rescues her. Birds of Prey (2020) corrects this decidedly stating Joker is a vicious boyfriend to Harley who soon leaves him, but in the comics Joker is known to stalk obsess over Harley even if she cuts ties with him, while he apparently just gives up on her in the film. Zack Snyder's Justice League flips it again as during the Bad Future segment, Joker while still unpleasant is shown to be a Retired Monster and The Atoner who has a shaky truce with Batman, which is actually genuine. In the comics and most media the Joker usually goes back to trying to kill/hurt Batman after the Enemy Mine ends.