To say Crisis on Infinite Earths was a huge undertaking is putting it mildly. The CW's latest superhero crossover was by far its most ambitious, drawing in dozens of heroes from all corners of DC's live-action multiverse, including a cameo from the DC movies' version of The Flash, Ezra Miller

Making Ezra Miller's Flash Cameo Happen

Loading

Crisis on Infinite Earths: Every Cameo in the Crossover 27 IMAGES

The Secrets of Earth-Prime in Crisis on Infinite Earths

Loading

Connecting The CW and DC Universe

Loading

This crossover wouldn't have happened without the guiding hand of executive producer (and longtime Arrowverse veteran) Marc Guggenheim. As the dust settles, we were able to chat with Guggenheim on the phone to learn more about what's next for the Arrowverse after Crisis on Infinite Earths. Below, Guggenheim sheds more light on how Miller's surprise cameo came to be and how the ramifications of Crisis will play out across all the shows.One of the most surprising things about Ezra Miller's cameo in Crisis on Infinite Earths: Part 4 isn't just that The CW was able to keep it a secret, but that Warner Bros. signed off on creating a direct link between the Arrowverse and the DC movie universe. Historically, WB Television has always been limited in terms of what DC characters it can and can't use. This cameo suggests the relationship between WB TV and the movies division has evolved, and that the TV side is working with more freedom than it has in the past."I think the relationship has always been good," said Guggenheim. "I think sometimes the Internet, because the Internet's always looking for conflict, it tries to make it out like we're at war with each other, we're angry, that we can't use Suicide Squad on Arrow or something, but it's like we're kind of old friends, and we'd go out and drink, and we are sort of bonded by our love for these characters."That being said, Guggenheim does feel the cameo may open the doors to more collaboration between the two divisions in the future."I would say that the relationship over the years has changed as the various corporate decisions and mandates have changed," he said. "I think what's exciting about bringing in Ezra is that it offers the hope of a closer collaboration or a closer synergy - that's probably a better word - with television and theatrical. It's just an exciting time. We were also talking after we shot Ezra's scene, and we were all just so jazzed by merging these two characters and seeing them riff off of each other. It just really excited all of us. I can't predict, and it's not up to me to decide the next steps forward, but it was something we were all so excited about."According to Guggenheim, there wasn't any pushback from WB theatrical regarding the content of the scene, including the twist that Miller's Barry Allen is inspired by Gustin's Barry to adopt the "Flash" name. If anything, Guggenheim said Miller himself was responsible for elevating the material and pushing it to a new level, comedically."Ezra and Grant ad-libbed some really fun material, like each of them are admiring each other's costumes," said Guggenheim. "It was there in the original draft, but it quite frankly just wasn't as funny as when they got on set and ad-libbed. Ezra, the way he's like, 'Oh, this is really comfortable,' and Grant's line... I think that the script just created the foundation for their interaction. Then Ezra and Grant took it the extra mile and really owned it."The creation of a new "Earth-Prime" is easily the most significant development in Crisis, as well as the one with the most far-reaching consequences. Where once shows like The Flash , Supergirl, and Black Lightning existed on separate worlds, all of The CW's DC shows now take place on the same Earth-Prime . That twist raises a number of interesting and tricky questions, questions which the various Arrowverse shows will address in the coming weeks and months, something Guggenheim refers to as "huge story potential.""Each show kind of expresses different elements of those continuity storylines differently," he said. "I'll give you an example, like the merging of the various Earths into Earth-Prime, that has a direct effect on the events of Legends of Tomorrow Season 5. You won't actually know exactly how that all works out until about midway through the season when we explain it to you, but you'll see that, basically, the events in Season 5 could not have happened if Earth-Prime hadn't existed."Guggenheim continued, "There's a really great twist coming up in Batwoman, for example. I think, appropriately, Crisis really does set up the Arrow series finale. The Arrow series finale, let's put it this way, would be extremely different if not for Crisis, but I think that would all be fair to say without spoiling anything."Perhaps the biggest difference between The CW's version Crisis and the original 1985 comic series involves the multiverse itself. The comic ended with the multiverse being eliminated entirely, leaving only one world and one group of heroes. The TV version, by comparison, effectively restored the multiverse as it was, with the only apparent changes being the creation of Earth-Prime and the old Earth-2 being replaced by a new version that will serve as the setting for DC Universe's Stargirl.While it goes without saying that Crisis was never going to end with the elimination of the DCEU or other fan-favorite universes like Smallville, we were curious if Guggenheim and his collaborators ever considered rolling the other Greg Berlanti-produced shows like Titans and Doom Patrol into Earth-Prime, creating a true Berlanti-verse. Guggenheim told us the idea was briefly toyed with early on, but it was never a serious consideration."We never strongly considered combining everything under a single banner, in large part because it's one thing for all of us 'Arrowverse shows' to be together because we're all in the same building, we're all working on the same network, we're all working together. All the other shows, they are often their own logistical pockets."Guggenheim continued, "I will say the showrunners of those shows were very, very gracious letting us have access to their characters and to pick up their characters and all of that, but we didn't want to dictate their future. That also goes for the cast. I can't remember - because it was so long ago that we first started talking about Crisis - but to the extent we ever discussed it, it was the most fleeting of thoughts."For more on the epic Crisis finale, check out our review for the final two episodes and see what Stephen Amell thought of the Crisis ending

Jesse is a mild-mannered staff writer for IGN. Allow him to lend a machete to your intellectual thicket by following @jschedeen on Twitter