It’s an event nearly 500 hours in the making when the “Arrowverse” and fellow CW superhero series “Black Lightning” collide in the five-part epic crossover event, “Crisis on Infinite Earths.”

Inspired by the iconic graphic novel by the same name, “Crisis on Infinite Earths” finds the “Arrowverse” heroes — including Green Arrow (Stephen Amell), The Flash (Grant Gustin), Supergirl (Melissa Benoist), Batwoman (Ruby Rose), Superman (Tyler Hoechlin), White Canary (Caity Lotz) and The Atom (Brandon Routh) — teaming up to try and save as many worlds in the multiverse as possible.

The event has, officially, been in the works for a year, when the writers of the “Arrowverse” discovered they’d be allowed to adapt “Crisis” while they were penning the 2018 crossover, “Elseworlds.”

“This was news that was met with incredible excitement by all of us,” executive producer Marc Guggenheim tells Variety. “We were like, ‘OK this is exciting because we know a year out what we’re going to be doing.’ It allowed us to really lay the foundation, not just in last year’s crossover, but really [in the individual shows] for the past year.”

But even with the prep time, the teams had a massive task to take on. Here, Guggenheim breaks down how “Crisis” (finally) came to fruition.

The Writing Process

Guggenheim calls their version of “Crisis” a “homage” to the comic title versus a beat-by-beat replay. (The original comic series killed off a number of the main heroes, but the television series are ongoing and will need to keep most of their players around longer term.) But despite not playing things out exactly the same, the team used five of the 12 comic book covers from the series to pitch the story to the CW and Warner Bros., with the intention of playing with those important moments — with their own twist.

In the lead-up to “Crisis,” “Arrow” has heavily leaned on its events, as Oliver (Amell) made peace with the fact that he had to sacrifice himself to save his friends; similarly, “The Flash’s” Barry (Gustin) tried to prepare Team Flash for life after he disappears post-“Crisis,” as was foretold.

Despite the individual series’ tie-in to the crossover, “it was very important to me that these showrunners maintained autonomy and authority over their shows,” Guggenheim says. “Very organically, it felt like ‘Arrow’ should lead up to ‘Crisis.’ That was that was just something that [‘Arrow’ showrunner] Beth Schwartz, the writers and I all just intuited. Similarly, ‘The Flash’ has been teasing ‘Crisis’ ever since their very first episode [when a headline proclaimed that The Flash went missing]. So they were also a very natural show to lay the pipe for ‘Crisis’ — moreso than ‘Supergirl’ or ‘Batwoman.’” (“Legends of Tomorrow,” a midseason series, kicks off its season with the “Crisis” finale in January 2020.)

When it came time to pen the five hours, Guggenheim assembled a writers’ room filled with the showrunners of “Arrow,” “The Flash,” “Supergirl,” “Legends of Tomorrow” and “Batwoman,” as well key additional writers from their teams for a two-week session.

“I came into that room with basically a scaffold — a series of big tentpole beats that could happen in each of the five hours — just so that we had something to work off of and have a basis for conversation,” Guggenheim recalls. “And knowing that every show has its own creative needs, as well as the difficulty, typically, in scheduling all of our various cast members, the scaffolding was was designed to be very modular. And it ended up pretty much we ended up adhering to it a to a surprising degree. What we ended up doing was, obviously, elaborating on it and adding more and more layers and dialing in various ideas.”

With five hours to play with (three airing in 2019, with the final two airing as a block in 2020), there was also the question of which order the shows should go in.

“When I worked out the tentpoles of the crossover, we knew that hour four was definitely going to be the ‘Arrow’ hour,” Guggenheim says. “We knew that hour five was definitely be the ‘Legends’ hour because in crossovers past, ‘Legends’ has always been the finale of the crossover. Which left hours one, two and three.”

“The CW said, ‘Look, if hour three could be ‘Flash,’ that would be great,” he continues. “But if you want to move it around, we’d be open to having a conversation.’ And when when I worked out those tentpoles, for a variety of different plot reasons, it made a lot of sense for the first hour to be ‘Supergirl.’ And this just worked best for the story, because even though we’re telling one singular story, there’s a variety of different production challenges and limitations that that will dictate whether or not a certain hour is best done by a certain production company. But ideally, when you’re watching all five hours, those little production maneuvers are invisible.”

After they hammered out the order and the big moments, the staffs retreated to their own rooms to break out the individual episodes. When they reunited, the makeshift room went over all five hours, moving beats as appropriate, and changing things that made later parts moot.

“It was a matter of getting all the different pieces to talk to each other,” Guggenheim says. “One of the things that’s really, really helpful is that we had writers on each of the different shows — for example, the ‘Supergirl’ writers would write a scene in the ‘Batwoman’ hour that featured Supergirl, so that we’re getting a consistency of voice across all the different five hours.”

Though the crossover was “designed to be a five-hour epic story,” the writers were also sure to give each title character their due.

“There are quintessential Supergirl moments in the ‘Supergirl’ hour; there’s quintessential Batwoman moments in ‘Batwoman,’” Guggenheim says. “Bruce Wayne, played by Kevin Conroy, makes his appearance in the ‘Batwoman’ hour just because that’s appropriate. Similarly, Flash-90, played John Wesley Shipp appears in ‘The Flash.’ But we never really talked about them in terms of this is the ‘Arrow’ episode; it was always this is hour four of the crossover.”

Worlds Collide

Making a cohesive story was, in many ways, the easiest part of the process, according to Guggenheim, as the crossover still had to be filmed in the shows’ quasi-normal (and overlapping) production window. Over the years of doing crossovers — both big and small — the teams adjusted how they handled things, from adding in additional production and shutdown days, to allow for the cast to move from series to series.

With a crossover this big, the team made the decision to focus on the characters that fans have known for upwards of eight seasons. But with an event requiring so much help from the outside, it was also natural to bring others in, as well.

Among those appearing in “Crisis” are “Smallville” stars Tom Welling and Erica Durance, Routh reprising his Clark Kent/Superman role, and Burt Ward.

“That [balance] was probably the the trickiest aspect here,” Guggenheim says. “It’s funny, every single day it has been a little bit of an emotional roller coaster ride as you hope and pray that this bit of casting is going to come through, or this Easter egg can happen.”

Guggenheim also admits there was a “financial limitation” to some of what they could achieve with guest cast because they had a “very limited budget for this kind of event” and “didn’t get additional money to bring in guest cast.”

Still, there was no shortage of interest from performers, he says, now that they have a track record of what these crossover events can accomplish. “There were some actors who approached us and made it known through their reps that they were interested in participating, and that was always awesome,” he says.

However, at less than a week until the episodes start to air (and with production wrapped), Guggenheim shares that they’re still working to get an additional person involved in the crossover: “The paint does not dry on this until we are literally about to deliver for air.”

Also in play? Which characters they got approval to use from DC Entertainment.

“We got approval on some, we didn’t get approval on all,” he says.

Arguably the biggest move is “Crisis” introducing Jefferson Pierce/Black Lightning (Cress Williams) from “Black Lightning” into the “Arrowverse” in the third hour of the event. And while the matters the characters are dealing with are very serious, his initiation into the group allowed for the writers to lean into more comedic moments. (The “Black Lightning” writers did a pass on Jefferson’s dialogue and reviewed cuts to make sure the character stayed consistent to their series.)

“I think probably the most fun that we had was writing little moments where Cress Williams’ character is reacting to this idea of there are other superheroes and, wow, some of them are really strange,” Guggenheim says. “He has a reaction to meeting Superman that I think is is really particularly humorous. And even the interaction he has with Batwoman, who sort of remarks, ‘Yeah, I was the new kid last year.’ We love our meta jokes in the crossovers.”

Despite the fun, deciding to bring Jefferson aboard ended up being easier said than done. With “Black Lightning” normally filming in Atlanta, Ga. (the crossover filmed in Vancouver), that show had to carve out a week in their own production for Williams to film his part of the episodes. The actor traveled on the weekends to maximize his filming time, but there were other things to fall into place, including traveling his stunt double and costume, Guggenheim notes.

“His costume is a very involved piece of equipment that has to be traveled [and] has a separate handler that has to travel with it. In fact, it was suggested to me at one point, by one of the money people, ‘Does Black Lightning really have to wear his costume in the crossover?’ He has to wear that costume. It just wouldn’t be the same if he didn’t.”

Keeping the Look and Tone Consistent

With the episodes filming nearly simultaneously, it was impossible for a single director to tackle the crossover themselves — and assembling their team was a task upon itself.

“We actually start off with hiring directors specifically for the crossover,” Guggenheim says. “And those directors are not just our normal A-list directors who do our finales and premieres: They are our A-list of directors specifically for crossover material, [due to their] experience with the ‘Arrowverse’ as a whole. They all have experience working on multiple ‘Arrowverse’ shows, so that helps us create a unified visual look.”

During production, the directors of photography from each show kept in contact to maintain consistency. On the post-production side, the teams worked to find a consistent look, finding a color palette, a title card, and font chyrons that are consistent throughout the run. And for “Batwoman,” which is normally filmed in a different aspect ratio than the other shows, the production team had to shift to the same ratio as its sister shows.

Additionally, “it’s all scored by Blake Neely and his team,” Guggenheim says. “In fact, we’re recording with a live orchestra, which is not something we typically do.”

Go Big or Go Home — The Stunts!

With a team-up this super, the expectation for the fights are taken to the next level. But with this many players on the field, they had to do things a bit differently.

“One of the things we did with the stunt sequences, particularly in hours one and four, is we did a lot of drone footage,” Guggenheim says. Using a first episode battle, in which the heroes are surrounded in an unusual way as an example, Guggenheim says they got were “literally flying the drone in between the combatants. So it’s not just choreographing the fight, you also have to add the drone into the mix, and the drone has to become part of the choreography.”

The high-stakes threat to the world(s) within the show also informed the nature of the fights.

“Our heroes have to team up to fight [in these episodes],” Guggenheim points out. That means the fights are “less fisticuffs like we do on ‘Arrow’ or ‘Batwoman’ and much sort of grander in scale and scope.” With that comes a “a huge visual effects component. We have to almost redesign the way we do the action sequences for the crossovers compared to what we do on each of the individual series.”

“Crisis on Infinite Earths” kicks off Sunday, Dec. 8 on the CW.