Arrow has certainly come a long way in 100 episodes.

The CW’s first Greg Berlanti-produced superhero series began as a gritty, realistic take on DC Comics’ iconic Green Arrow character. Back in season one, Oliver Queen (Stephen Amell) was just a parkour-loving expert archer taking down corrupt businessmen. But now, as Arrow hits its 100th episode during the gigantic three-part four-show crossover with Supergirl, The Flash and Legends of Tomorrow, Oliver will find himself up against an alien invasion. That’s a lot for a former billionaire party boy to handle.

“It was Greg’s idea to do the Dominators,” executive producer Marc Guggenheim told a small group of reporters after a screening of the three “Invasion!” episodes. “I don’t want to speak for Greg, but I think the way it came out was we all collectively wanted the superheroes to face an external threat. And by external, I don’t mean extraterrestrial; I mean not a Big Bad from one of the shows. Rather, a threat that came from outside of the shows.”

While the showrunners could have picked any number of DC Comics villains for Oliver, Barry Allen (Grant Gustin), Kara Danvers (Melissa Benoist), Sara Lance (Caity Lotz) and the rest of the costumed superheroes to fight, they decided to bring to life the alien race of the Dominators.

“Greg just walked in one day and was like, ‘Let’s do Invasion!‘” Guggenheim said. “I think [executive producer Andrew Kreisberg and I] both had the exact same twin reactions: ‘That’s totally awesome,’ and, ‘Oh my god, how the hell are we going to do that?’ I’m a big fan of that particular series and I’m a big Bill Mantlo fan. That was Bill Mantlo’s first work for DC Comics after decades at Marvel, so that particular series looms very large in my personal estimation.”

But fans of the Invasion! comic book story might be disappointed in how it was adapted for the crossover episodes, as most of the comic book source material about the Dominators had to get cut for time.

“There were a lot more nods to the comic book,” Kreisberg said. “There was discussion about the size of their circles and how that was indicative of where they stood in the caste system, so I’m sorry some of that stuff went away just because it was great flavor, and I think people who are fans of the original comic book would have seen a lot more of our joy at adapting Invasion! in there. But it just, unfortunately, to get these down to the proper time, that’s the kind of stuff that tends to fall by the wayside.”

Guggenheim added,”Including the robes. Originally we had designed the Dominators with the green robes that were very iconic of the comic. When we decided to do all the Dominators full CG as we ended up doing, the robes had to go away because the presence of the robes made animating them prohibitively impossible. But we’ve got these cool designs of like, ‘Oh wow, they really look like the Dominators of the comics.’ It’s just stuff like that. Andrew’s right; that was the first stuff that had to go for time.”

And yes, the showrunners know that an alien invasion is not what Arrow fans are used to seeing from a villain.

“What’s so nice about the shows, for us, is each of the show has their own identity,” Kreisberg said. “Especially this season, it feels like every show has sort of doubled down on what it’s about. With Arrow, we’ve really returned to the sort of gritty crime drama that it originally was and there’s so many echoes in this season to the earlier seasons. Flash has the metahumans, Legends have time traveling, Supergirl has doubled down on the alien aspect. I think that’s what makes the shows feel different, so you’re not watching four hours of the same stuff every week. You’re really getting a different flavor and different world. Whereas now that possibility is open because they’ve faced aliens, there’s certainly no plans to have an ongoing or bring up an alien storyline on Arrow. That really feels like the purview of Supergirl.”

Just a few weeks before the crossover aired, the showrunners took many fans and critics alike by surprise when they announced that the big crossover event, while made up of four shows, would only be three episodes long.

“Part of what made it a little bit difficult was all the other shows—Flash, Arrow and Legends—all have a subsequent episode to the crossovers. But with Supergirl, it was our midseason finale, so we wanted to make sure that episode eight of Supergirl spoke to what had been happening on the first seven episodes of Supergirl and not just as a tie-in. And then, just logistically speaking, last year we did two shows with a sprinkling of Legends, and this year, we went to three shows. The idea of going to four full shows, I’m not sure we could have pulled [it] off, logistically speaking.”

Another conscious choice that the showrunners made was scheduling the crossover to take place when Arrow hit 100 episodes, instead of separating the two milestones into different episodes.

“That was the challenge was just how much to service the 100th episode-ness and how much to service the crossover,” Guggenheim said. “I think it’s pretty obvious that we chose to go 100th episode, but it was totally a challenge. It was Greg’s idea to have our five Arrow people and basically enter them into… we call it the matrix, lowercase ‘M.’ The Dominator matrix. And that allowed us to have our cake and eat it too.”

The Arrow episode of the crossover finds Oliver, Diggle (David Ramsey), Thea (Willa Holland), Ray (Brandon Routh) and Sara captured by the Dominators and trapped in an alternative reality where Oliver never got on the Queen’s Gambit and therefore never became the Green Arrow.

“This was sort of Oliver’s version of Flashpoint and it’s a little bit of Oliver’s version of ‘The Girl Who has Everything’ from last year’s Supergirl,” Guggenheim said. “There’s a reason why these stories are iconic or familiar tropes even in comic books. It’s because when you show the protagonist the path not taken, you basically put them in the situation where they can choose to stay on that path or go back to their life with all of its ugly aspects and challenges.”

The biggest challenge when it came to developing the Arrow crossover episode? “Getting the cast,” according to Kreisberg. “Getting Jamey Sheridan [to return as Robert Queen], getting Susanna Thompson back [as Moira Queen],” Kreisberg said. “We tried desperately to get Colin Donnell—”

Guggenheim added, “And Colton [Haynes].”

Kreisberg continued, “We just couldn’t pull it all off,” speaking in reference to Tommy and Roy’s absence from the episode. “I was just telling Marc, the show’s in syndication now and I caught a couple of the early episodes on TNT and we’re so immersed in it, sometimes it’s easy to forget just how far the show has come and how different it used to be. Watching the early episodes and watching the 100th and seeing Oliver have scenes with his mom and especially the scene with Thea and Moira, and remembering how important [Moira] was to the mythology and these characters and to everything that happened, that was the thing that made it feel like a 100th episode. Our first mission was to make sure we lock in those cast and we were fortunate that everybody who has left the show was so excited to come back.”

Because of the huge list of cast members for each of the crossover episodes, not even including special returning guest stars, the only show that was able to have a table read of the script was actually Arrow.

“We were able to have Jamie, but we weren’t able to have anybody else,” Guggenheim said.

What are you most excited to see from Arrow‘s 100th episode? Tweet me your thoughts at @SydneyBucksbaum!

Arrow airs Wednesdays at 8 p.m. on The CW.

Images: The CW

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