Arrowverse fans have known for nearly a year that a crisis is coming -- and tonight it finally arrives: the crossover event "Crisis on Infinite Earths." Since its announcement at the end of last season’s Arrowverse crossover, "Elseworlds," audiences have searched for clues and pieced together details based on a steady stream of casting announcements. Yet, a press screening of the first two parts of the crossover proved nothing has prepared fans for what’s to come.

In a Q&A following the screening, five of the Arrowverse’s showrunners -- and “Crisis” masterminds -- discussed the challenges of making sure it all came together, and what fans can expect when the event finally begins. The session featured “Crisis on Infinite Earths” showrunner Marc Guggenheim, Arrow showrunner Beth Schwartz, Supergirl showrunner Robert Rovner, Legends of Tomorrow showrunner Keto Shimizu and Batwoman showrunner Caroline Dries.

RELATED: Crisis: Supergirl and Superman Mourn [SPOILER] in New Clip

"Crisis on Infinite Earths" was a massive undertaking involving all five of the current Arrowverse shows as well as many guest stars, but the showrunners were especially invested in telling a satisfying story. As a result, the group started work on the crossover early in the season. Legends’ Shimizu explained, “We started before production on any of our shows had actually started. And thank goodness we did, because it wouldn’t have been possible if we had started it too late.”

Guggenheim also noted that because of the early start on the crossover, the showrunners didn’t always know exactly what would be happening in the episodes that led into it or what would happen afterward. “So there was a little bit of educated guess work on our part on both ends," he said, "both in terms of breaking out the story and kind of having a sense as to where all the shows would be as well as each of the shows leading up to it.”

“'Crisis' is essentially a prequel to [Legends of Tomorrow] Season 5,” Shimizu added of the series, which doesn't start its latest season until January. “However, it was shot out of order, so we have to know the sequel to everything that was going to happen in 'Crisis' before we had even written 'Crisis.' So that was very tricky for us.”

Meanwhile, Arrow, with its truncated 10-episode eighth and final season, was working toward wrapping up. “Arrow was actually the opposite [of Legends]," Schwartz said, "because we were going towards the end of our series and season, so it’s different than the crossovers have been in the past.”

While the crossover presented many challenges, showrunners wanted to make sure they met fan expectations after almost a year of anticipation. To do that, they left room for the characters to share meaningful, emotional moments amid the action.

“It needed to feel emotionally significant for everybody,” Shimizu observed, “and I think, yes, you can have all these incredible cameos from all these fun, different versions of these DC characters that we know and love so much, but at the core of it we really wanted to keep track of the [main characters] from all of our shows and have them go through something really incredibly significant for their characters in this crisis.

"And I think that honestly is what grounds it and makes this 'Crisis' important on a very deep, heart level. You feel for these characters as they’re going through this amazing adventure together and it brings them all closer together, which I think is sort of the definition of epic in terms of the relationship that all these shows have with one another.”

Guggenheim agreed. “Those are the scenes that really make it live,” he said. “And, yes, we’re going to throw as much visual effects and bombast as we can.”

“There’s still crazy-ass battles,” Shimizu promised.

RELATED: Crisis on Infinite Earths Has At Least Six Unannounced Characters

The showrunners also elaborated on how the crossover impacts their individual shows moving forward. Needless to say, each and every Arrowverse show will experience profound changes following the 'Crisis.'

On Legends, Shimizu said, “As our prequel, the crossover sets up our season, both mythology and Sara Lance’s trajectory coming out of this. This hits her pretty hard, in a good way.”

Meanwhile, Rovner teased, “It really resets a lot of what’s happening on Supergirl in the second half [of the season]. That was another thing that was challenging about this crossover that’s different from the other [season’s crossovers] is that… it impacts everything going on on the show. It’s fun to see what happens afterwards.”

Meanwhile Dries noted, “We have some fallout on Batwoman. It’s a little tricky because our characters aren’t yet exposed to this notion of multiple universes and superheroes and powers and stuff so there’s a fine line but I think it will have… a really shocking resonance.”

While The Flash showrunner Eric Wallace wasn’t at the Q&A to speak to the crossover's impact on his show, Guggenheim revealed, “During the 'Crisis,' Barry will do something that is very significant that will have ramifications for the remainder of Season 6 [of] The Flash.”

And finally, although Arrow will only have two episodes remaining in the series following the crossover. “['Crisis'] changes literally everything on Arrow,” Schwartz said.

Crisis on Infinite Earths feature

While fans of the Arrowverse will no doubt tune in for the event, Guggenheim was also quick to note that those who were curious about the crossover but hadn’t spent much time with the Arrowverse or who only watched one of the Arrowverse shows would be able to enjoy 'Crisis' too. “You know what’s funny… [network executives] were of the opinion, and we agreed, that you kind of don’t need any sort of set up [for the crossover].

“It’s fun to see last year’s crossover and meet the Monitor for the first time and know that a 'Crisis' is coming and that Oliver made this bargain with the Monitor…. But you get all that information in the body of the story [of the five episodes]. So you’ll notice one thing we don’t do for broadcast is we’re not having any recaps. The only recap we’ve cut for the crossover is going to be at the beginning of hour four, which’ll be in January, because we kind of feel like you can dive right in.”

Meanwhile, those who are familiar with the comic book version of "Crisis on Infinite Earths" can look forward to some Easter eggs acknowledging the source material. Although the story in the television crossover is firmly set in the Arrowverse and fans don’t need to be familiar with the comics to understand it, Guggenheim noted the showrunners were mindful to include nods to its comic origin.

“One of the things we did when we pitched this to the network and the studio was, it’s five episodes, and we picked five of the 12 comic book covers,” Guggenheim explained. “'And we said in this episode you’re going to get this and in this episode you’re going to get this.' And, again, it’s our version of it, but it’s definitely meant to be evocative of the original comic.”

"Crisis on Infinite Earths" begins Sunday, Dec. 8 at 8 p.m. ET/PT on Supergirl, then continues in Batwoman on Monday, Dec. 9 at 8 p.m. ET/PT and in The Flash on Tuesday, Dec. 10 at 8 p.m. ET/PT. After the winter hiatus, the crossover will conclude on Tuesday, Jan. 14 in Arrow at 8 p.m. ET/PT and in DC's Legends of Tomorrow at 9 p.m. ET/PT.

NEXT: Supergirl's David Harewood Teases His 'Significant' Role in Crisis