When The Flash and Lex Luthor swapped minds in "The Great Brain Robbery" the bodies kept their voice actors, so now Lex Luthor is being voiced by Michael Rosenbaum. Michael Rosenbaum played the role of Lex Luthor on Smallville.
The entire Royal Flush Gang from "Wild Cards" was played by the cast of Teen Titans. Hynden Walchnote who played Starfire and Tara Strongnote who played Raven switched roles, with Strong being the attractive, popular one (Queen) and with Walch being the quiet, darker one with the emotional issues (Ace). The boys all played the same roles, the leader has the voice of Robin, the shapeshifter has the voice of Beast Boy, and the big guy has the voice of Cyborg.
In "The Ties That Bind", Virman Vundabar is voiced by Arte Johnson, and gets to say his Catch Phrase ("Very interesting") from his old show.
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 series, as were all Teen Titans characters. 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 (they were trying to establish the comics as a non-parody group, which the episode would have been counterproductive) and the show had to come up with the expies that were used in the episode proper. This teaser shows the animated JSA.
Fake American: Jennifer Hale, who voices Zatanna, Killer Frost, and Giganta, is Canadian, though with American parents and raised in America.
Amy Acker, best known as Fred and Illyria, was the Huntress. Giselle Loren, who voiced Buffy in the video game of Buffy the Vampire Slayer was Stargirl. Alexis "Wesley" Denisof was Mirror Master. Juliet Landau was Tala, it's like the writers watched both series and picked and chose their favorites for the roles.
Gary Cole is Senator J. Allen Carter in "Secret Origins".
In "In Blackest Night", the prosecutor is Kurtwood Smith, Kanjar-Ro is Rene "Odo" Auberjonois taking a turn at breaking the law rather than enforcing it, and the head Manhunter is James Remar.
And later, James Remar would voice Hawkman and Shadow Thief and—though The Other Darrin as far as the DCAU is concerned—Rene Auberjonois would reprise his role on Super Friends as Dessad.
Hawk and Dove are voiced by Fred Savage and Jason Hervey, previously known as Kevin and Wayne Arnold on The Wonder Years. Except Wayne's playing the pacifist now; originally, their roles were supposed to reflect their roles on The Wonder Years, but the actors decided to switch.
"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.
Series regulars Phil LaMarr (Green Lantern) and Maria Canals (Hawkgirl) replaced respectively Michael Dorn and Lori Petty as Steel and Livewire, also respectively (though Dorn did reprise his role as Kalibak).
When Metallo turned up in "Hereafter", Corey Burton voiced him. McDowell was back, though, in "Chaos at the Earth's Core".
Old Shame: After the events of the Thanagarian Invasion, Shayera considers her original costume and even the name "Hawkgirl" to be this, changing into a new, more down-to-earth outfit and referring to herself only by her real name.
Post Script Season: The third season of JLU. Notice the frequency of more out-there episodes and characters.
Phil LaMarr as John Stewart/Green Lantern. He proudly boasted that for "In Blackest Night," he didn't need his script when reciting the Green Lantern oath.
Phil Morris was a huge comic book fan and didn't need to be told anything about Vandal Savage before he voiced him.
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 super group (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!
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.
Hawkgirl and Livewire are both voiced by Maria Canals-Barrera.
Dr. Moon's only speaking role was interrogating the Combs-voiced Question.
Mark Hamill voiced both Joker and Solomon Grundy in Injustice for All, and of course they start arguing.
Throw It In: 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.
What Could Have Been: Numerous plots and ideas were dreamed up by the writers, and some even went to the scripting and storyboarding stage, before they had to be abandoned for one reason or another before the actual episodes were produced. These include, but are not limited to:
The Justice Society of America was planned to appear in "Legends", which was an Affectionate Parody of The Golden Age of Comic Books. This idea went so far as to have character designs and promotional stills produced, but they were forced to transform the Society into the Justice Guild of America by DC Comics, who said they could not use the original characters. In the defense of DC Comics, they were trying to establish the Justice Society comic ongoing at the time as a serious story, which the affectionate parody nature of the episode would not have helped.
The Crime Syndicate, a not-quite-Evil Twin version of the League from a parallel world, was scheduled to appear in two separate stories, but both were abandoned for different reasons.
The first was "A Better World," an episode where the Syndicate was replaced by the Justice Lords, a more direct mirror image of the League which featured alternate versions of the same characters, rather than villains who were merely similar in theme and power.
The direct-to-DVD film Justice League: Crisis on Two Earths is based on the film that was originally supposed to bridge the gap between Justice League and Unlimited.Justice League: Worlds Collide would have explained the origins of the expanded roster, the new Watchtower, and Wonder Woman's invisible jet and would have featured the Crime Syndicate as the primary villains. Worlds Collide was never produced as such, but people on the Crisis on Two Earths production team have said that the film stays within "95 percent" of the original script, and indeed it shows things like the second Watchtower under construction, the beginnings of an expanded Justice League, subtle evidence that Hawkgirl recently left the League, and the origin of Wonder Woman's invisible jet (which made its first DCAU appearance two episodes into Unlimited).
The Birds of Prey would have starred in an episode of Justice League Unlimited. In a semi-retelling of the origin of Oracle and the team there was originally a plan for Batgirl to be hospitalized in an episode and, unable to finish the mission she was on when she was injured, she would contact Huntress and Black Canary to help her finish the job while supervising them over their radios. The "Bat-Embargo," which was a moratorium on using certain Batman-related characters after the premier of The Batman and the production of Batman Begins, meant that Batgirl had to be written out of the episode and the story was re-written and expanded into "Double Date". The episode still featured Huntress and Black Canary, but also included their "dates" The Question and Green Arrow.
The commentary for "The Return" reveals that, when "Starcrossed" was scripted, the writers did not know where Shayera had gone after leaving the League. One theory was that she had become the resident guardian angel of a small South American village, and another was that one of the victim planets of the Thanagarians had found her seeking revenge (this story was later used in the Unlimited episode "Hunter's Moon"). Eventually they decided that she had sought sanctuary with Doctor Fate, and this led to the double meaning of the title "The Return".
Lastly, a minor cameo instead of a plot-important appearance, the writers originally wanted a Joker cameo in "This Little Piggy" for a brief moment of comic relief. After Diana had been turned into a pig, Batman would be walking down the street with her and would pass the Joker preparing to commit a crime with his gang. The Joker, in the middle of preparing a heist and laying out his convoluted plan to deal with Batman, would see Batman holding and consoling a pig and would stare at it in Dull Surprise, then throw his hands in the air and abandon the entire scheme. Sadly, this scene (which might very well have been the single greatest scene ever animated), had to be dropped from the episode for a variety of reasons, including time and pacing problems.
Tim Daly was originally set to reprise the role of Superman from Superman: The Animated Series. He did do some initial recordings, but he was not available due to his commitment to the TV remake of The Fugitive.
Captain Marvel was originally supposed to appear in Hereafter as Superman's replacement, but they went with Lobo instead.
Word of God: The whole Amazo and Grundy thing was supposed to be a Brick Joke in the last episode but they forgot. They were going to include a gag in the finale showing Amazo sitting on an asteroid somewhere, wondering whether or not it was safe to return (even though his omniscience should've told him so).
While writing the Cadmus arc they seriously considered having Batman defect from the Justice League and join Cadmus after being convinced that Cadmus was right about his god like super-friends, but they nixed the idea since they felt it would have required making the League look and act like a bunch of jerks or make Batman look and act like a jerk (and he had already crossed the Jerkass line before in season 2).
That basically what happens in Marvel Comics' Squadron Supreme mini-series from the 1980s featuring an alternate reality Captain Ersatz version of the JLA. The Squadron decide to create a Utopian society on their world but at the cost of individual liberty. One of their members, Nighthawk(their world's version of Batman) forms his own team to stop them. As a result both sides do end up looking like jerks.