Actor-Inspired Heroism: The writers initially wanted Vixen/Mari Mc Cabe to be a very temporary Romantic False Lead for Green Lantern/John Stewart. She was supposed to reveal herself as a big enough jerk that John could break up with her guilt-free and get back with his first love interest, Hawkgirl/Shayera Hol. But Mari's VA Creator/Gina Torres played her with enough humanity that the writers couldn't bring themselves to make her a jerk. As a result, the writers couldn't come up with another in-character reason for John to break up with her so their relationship lasted through the end of the series (with some ambiguous clues that John might get back together with Shayera sometime in the future).
Approval of God: Dave Gibbons voiced his approval of the episode Justice League Unlimited S1E2 "For the Man Who Has Everything", including the necessary changes made in adaptation. Contrary to rumor however, Alan Moore is not known to have voiced his opinion on it one way or another, but he notably allowed DC to use his name in the credits (which is not something that he's known for doing). More information here.
Edward Asner as Granny Goodness, a role he had previously played in Superman: The Animated Series and would reprise in Superman/Batman: Apocalypse. A Bruce Timm artbook revealed that Granny Goodness was specifically designed to look like Ed Asner in drag... so as a Casting Gag they figured they should get Asner and just have him speak in a deliberately bad falsetto. (Visually and audially, Asner is an excellent choice to play an evil old battleaxe.)
Edited for Syndication: JLU found a new home on the Saban branded Vortexx block. However, it suffered from Saturday Morning edits. From the first episode, guns were given laser sounds, Green Arrow's boxing glove arrow hitting someone was cut, and Black Canary seductively zipping up her boot was cut.
After The Batman began airing during the Unlimited phase, all major characters from that property other than Batman himself were suddenly verboten in JLU; this was referred to as the "Bat-Embargo" by fans. Similarly, the villain Black Manta was banned when Aquamanseemed a go for a live-action series, as were Robin and Nightwing (aside from a silhouetted cameo from the latter in "Grudge Match") because of Teen Titans (which resulted in Wonder Woman taking Jason Todd's role as the one who brought down Mongul in the adaptation ofFor the Man Who Has Everything). Fortunately, DC has plenty of characters to use. As a result, viewers got to see more minor and obscure characters, like Deadman, Warlord, and the Seven Soldiers of Victory, who otherwise would have probably been ignored.
The episode "Legends" was originally supposed to feature the JSA, but DC management informed the show's producers that they could not use the JSA (one: they were trying to establish the comics as a non-parody group and two: producer James Tucker later revealed on Twitter in 2020 that a sticking point was the JSA dying, both of which the episode would have been counterproductive towards) and the show had to come up with the expies that were used in the episode proper. This teaser shows the animated JSA. However, most would agree not having the real JSA made the episode better.
Hro Talak was originally supposed to be Katar Hol, the Silver Age Hawkman and Shayera's Distaff Counterpart in the comics. However, DC didn't allow the showrunners to make Hawkman evil. So they created Hro Talak, whose name is an anagram of "Katar Hol". Later, the show introduced a proper Hawkman based on the Golden Age version, a human named Carter Hall - who in a nifty twist claimed to be the reincarnation of Katar Hol, so the veto turned out for the best.
Fake American: Jennifer Hale, who voices Zatanna, Killer Frost, and Giganta, is Canadian, though with American parents and raised in America.
Flip-Flop of God: Dwayne McDuffie confirmed that yes, Darkseid and Luthor are trapped in the Source Wall and might as well be dead, but later retracted that statement in 2011, saying that they escaped.
Lying Creator: Knowing the series bible would be used for promotional material, the crew created a fake bio for Hawkgirl to hide the truth of her arc in the first two seasons.
"In Blackest Night" is about GL on trial for the destruction of a planet in a nod to Cosmic Odyssey. It has nothing to do with zombie Lanterns.
The name is much older than either story- it comes from a) the Green Lantern Oath ("In brightest day, in blackest night...") and b) an old Alan Moore story where one of the Five Inversions (demons) prophesizes the end of the Green Lantern Corps.
In-universe, the hero "The Question," is given the nickname "Q." This does not mean he's a mischievous omnipotent being known best for tormenting French Starfleet Captains. Although, given Q's ability to look like anything or anyone, and his myriad abilities He also has a similar fashion sense to Q from Street Fighter.
Jeffrey Combs does his own singing as The Question, although it is Played for Laughs. Still kind of good though.
Played straight with Green Arrow humming his own theme in "To Another Shore", as Kin Shriner's attempt didn't work for the creators so producer Bruce Timm took a crack at it and was still unhappy, so it was dubbed in looping by Lex Lang.
Series regulars Phil LaMarr (Green Lantern) and Maria Canals Barrera (Hawkgirl) replaced respectively Michael Dorn and Lori Petty as Steel and Livewire, also respectively (though Dorn did reprise his role as Kalibak).
In the first season, the producers held back the airing of "Injustice For All" for eight months, so that it would coincide with the Video Game with the same name. But since it did air elsewhere during that period, it didn't take long for spoilers and pirated copies to appear online. However, both "Fury" and "Legends" have have elements that didn't appear until "Injustice for All".note "Fury" has its main villain gather the Injustice Gang sans Luthor, Joker, Cheetah, and Ultra-Humanite, with Shade and Copperhead commenting that she's a better boss than Luthor and "Legends" opens with Luthor in the Powered Armor designed to save his life controlling a mecha against the League.
In the second season, everything between "A Better World" and "Starcrossed" has their airing order rearranged from their production order. "The Terror Beyond" and "Eclipsed" had their airing order swapped from their production order, as did "Hereafter" and "Secret Society", and "Comfort and Joy" and "Wild Cards". However, this doesn't really affect anything, even with Green Lantern and Hawkgirl's Relationship Upgrade in "Wild Cards" despite "Comfort and Joy" being the episode before it in production order, but airing after.
Playing Against Type: Jennifer Hale as Giganta, a more explicitly "girly" character than the tomboys she usually plays. Pronounced due to her playing Killer Frost as well.
Post-Script Season: The season two finale of Justice League was written as a potential Grand Finale, making JLU something of a post-script Sequel Series. The second season of JLU was also intended to be the wrap-up, hence the Book-Ends with "Epilogue". After wrapping up so many storylines, the only place left to go was the Legion of Doom. They also took the opportunity to tap into the deepest pools of the DCU, including Hawkman, The Warlord and Deadman. According to Dwayne McDuffie, there were plans for a fourth season if Warner Bros. wanted them to do one.
Michael Beach, who voted Mr. Terrific and Devil Ray in the final season, voiced the male rock climber and one of the soldiers in "Dark Heart".
Recycled Script: Earth's mightiest heroes are suddenly lost in another dimension. Not all of them: the most powerful one and the Badass Normal were left behind. The lost heroes thought for a moment that they were in the past, but not, according to the newspapers the date is right. But some brief visions of doom tell them that something is not right. They soon find out that they are in another dimension, home of another supergroup (a Captain Ersatz of another group, which can not be used directly for network reasons). They fight each other first, then become friends, and fight against the real foe: a kid with a big and grotesque brain, so powerful that he actually created and controlled the world surrounding them. Sounds familiar? Hope so, true believer! That's the plot of The Avengers#85 and #86, the debut of the Squadron Supreme!
Refitted for Sequel: Back when there were considerations to have Superman: The Animated Series be about Superman teaming up with various guest star heroes (and forming the Justice League at the end of the first season), some of these heroes included Green Arrow, Black Canary, Hawkman, Hawkgirl, the Question, Vixen, and Mister Miracle, who would later appear in Justice League.
The show consistently put out some of Cartoon Network's best ratings in spite of the fact that it was rarely advertised, chronically shoved into unusual timeslots (six in the evening on a Saturday?!), and only aired once a week (most of Cartoon Network's other shows at this time aired reruns several times a week).
When the last season premiered in September 2005, Cartoon Network aired new episodes in an hour timeslot on Toonami. So, four episodes out of the thirteen ordered were burned off in two weeks... and new episodes didn't resume until the following February. And in-between, reruns were non-existent.
It was implied that Cheetah was killed by Solomon Grundy in "Injustice For All", but when the animators mistakenly drew her being arrested at the end of the episode, it was simply easier to say she was alive.
One of the most notable differences is having Wally West as the Flash, with Barry Allen only given a brief mention as Wally's unnamed uncle, though Wally being a Legacy Character to him was obscured or omitted. After the show aired, Barry Allen was brought Back from the Dead and has been the main Flash again in the comics and all adaptations to date.
Similarly, John Stewart (and Kyle Rayner before him, in Superman: The Animated Series) was the main Green Lantern with no Hal Jordan in sight, until he appeared as explicitly an alternate-timeline anomaly for a few minutes. Hal returned to life in the comics while the show was airing and, similar to Barry, has become the main GL again everywhere ever since. However, the show chose to make John its main GL instead of Kyle as in the comics at the time and is directly responsible for John's renewed presence in the comics ever since.
The expansion of the Green Lantern mythos with variously colored Corps came after the show and is nowhere to be found here, while it is an integral part of the mythos now (and reflected in the later Green Lantern: The Animated Series, which adapts the Timm Style into 3D CGI animation despite not being part of the DCAU). Sinestro in particular is depicted as his classic version wearing blue, while his revamp as head of the Sinestro Corps, with "Yellow Lantern" suit to match, was so popular that it's hard to imagine him as anything else now.
I don't think he'd be happier on Krypton. It's just that Superman never really knew Krypton and had a profound longing to experience that part of his history. Krypton is part of him but Earth, I think, is a bigger part.
The late Dwayne McDuffie more or less retconned or at least clarified Darkseid's resurrection in "Alive!" via this on his message board and on the special features on the DVD set for the final season, revealing Tala purposely resurrected Darkseid as a final "Screw you!" to Luthor, rather than it being an accident.
You Look Familiar: Phil Morris voiced the gorilla who interrogated Batman, Wonder Woman, Martian Manhunter, and Hawkgirl in "The Brave and the Bold" before being cast as Vandal Savage. The same episode had Dr. Sarah Corwin voiced by Virginia Madsen, who'd go on to voice Roulette.