SPOILER WARNING: This article contains spoilers for the 100th episode of "Arrow," which as of publication has not finished airing on the west coast.


As things turned out, the "Arrow" installment of The CW's "Invasion!" crossover between its four DC Comics-based superhero shows also happened to be the 100th episode of the series that started the "Arrowverse" back in 2012. So for tonight's episode -- also titled "Invasion!" -- the show's writers and producers had to juggle both paying tribute to the show's history and moving the larger story forward.

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The show's plan to do that was to have five past and present Arrow characters -- Green Arrow (Stephen Amell), Diggle (David Ramsey), Thea (Willa Holland), White Canary (Caity Lotz) and the Atom (Brandon Routh) -- abducted at the end of last night's episode of "The Flash" by The Dominators, the alien race and main antagonists of both the TV "Invasion!" and the 1988-1989 DC Comics story that inspired it. On The Dominators' ship, the five characters are held in pods -- think "The Matrix" -- and experience a shared hallucination in a world where Oliver Queen never became the Green Arrow, and instead lives a peaceful existence with a still-alive Laurel (Katie Cassidy, pictured above with Lotz) as his fiancee.

As part of these scenes, many past "Arrow" cast members returned to the show -- along with Katie Cassidy, Jamey Sheridan and Susanna Thompson, who played Oliver's parents Robert and Moira in earlier seasons, appear. According to "Arrow" executive producers Marc Guggenheim and Andrew Kreisberg, the show "desperately" tried to get Colin Donnell (Tommy Merlyn on "Arrow" season one) and Colton Haynes (Roy Harper/Arsenal from the show's first three seasons) to return, but the actors' schedules didn't allow it -- though they do appear in the episode, courtesy of a vision Oliver has when he sees what he's leaving behind as he chooses to flee his (considerably more comfortable) Dominators-induced hallucination and return to the real world.

"I was writing that scene where Oliver goes back, and it just popped into my head that, basically all the actors we can't get, this is a way we can get them," Guggenheim told reporters including CBR during a Q&A this week at The CW's Burbank offices. "Props to our visual effect house -- Zoic [Studios] handled those shots, and they did an amazing, amazing job, particularly with Colin and with Colton. We couldn't reshoot them, we had to take them from old episodes and roto them out and put them into this. It was hard, obviously, because they had to work with preexisting footage. And they did an incredible, incredible job."

For Kreisberg, it was a reminder of how far the show -- which started relatively grounded, despite the superpower-filled "The Flash" and the time travel-centric "DC's Legends of Tomorrow" both spinning out of it -- has come in its four and a half seasons.

"The show's in syndication now, and I caught a couple of the early episodes on TNT," Kreisberg told the gathered press. "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 those early episodes, and then seeing the 100th, and seeing Oliver have scenes with his mom, especially the scene with Thea and Moira, and remembering how important she was to the mythology and to these characters to everything that happened, that was the thing that made this feel like a 100th episode."

"Arrow" airs 8 p.m. Wednesdays on The CW; the final part of the "Invasion!" crossover, an episode of "DC's Legends of Tomorrow," airs 8 p.m. Thursday.