[Contains spoilers for Arrow's "Invasion!" crossover episode.]
After being abducted by Dominators at the end of The Flash's Invasion! episode, Arrow picks up with our heroes being brainwashed by the mysterious and hostile aliens. While lost in this dream state, Oliver, Sara, Diggle, Ray, and Thea wind up in a shared alternate reality. Similar to an episode of Supergirl from season 1, "For the Girl who has Everything," Ollie struggles to fight his way out of this giant "what if" scenario so he can get back to helping the rest of the heroes take down the real threat.
The 100th episode is quite a milestone for any network show, signifying (at least) five years on air. Everyone involved with Arrow, from the cast to producers and even The CW, has been especially pumped about this special episode for both the sheer number of past and present cast members who returned, and as a pivotal part of the massive four series DCTV cross-over event. "Invasion!" has nods all the way back to the pilot episode and brings back fan favorites like Moira Queen and Deathstroke.
For all the faces who did return, there were two major characters who, due to various other actor commitments, couldn't actually film an appearance. According to CB, producer Marc Guggenheim didn't let a silly thing like a physical presence stop them from getting their airtime in the 100th episode. Despite having "tried desperately to get Colin Donnell and Colton Haynes. We just couldn't pull it all off," Guggenheim said. Of their brief hologram appearances, he added:
"Honestly, I can't even tell you where the idea came from. It was just, I was writing that scene where Oliver goes back and it just popped into my head that you know, basically all the actors that we can't get, because at the time I didn't know who we'd be able to get and who we couldn't... The [visual effects team] did an amazing job, particularly with Colin and with Colton because we couldn't re-shoot them. We had to take them from the old episodes and roto them out and then put them into [the 100th episode]."
Both Donnell and Haynes had short, but important, arcs over the course of Arrow, so it's understandable why Guggenheim felt it so important that they be featured in this milestone. Though there was only got a glimpse before Oliver snapped back to reality, the moment was sincere enough that longtime fans could appreciate the gravity of all those characters staring back at him.
The sentimental value of Arrow going "back to its roots" even in the midst of a cross-over is something that producers felt took precedence. Guggenheim also said that the importance of bringing back cast members was always the driving force for this 100th episode, saying:
"That was the thing that made this feel like a hundredth episode. That was our first mission, was to make sure we locked in those casts. We were fortunate enough that everybody who's left the show was so excited to come back."
The trip down memory lane was a nice change of pace for Arrow, but it's back to business as the four-part event, "Invasion!," concludes with Legends of Tomorrow on December 1st.
Supergirl airs Mondays at 8pm on The CW; followed by The Flash on Tuesdays in the same time slot; Arrow on Wednesdays at 8pm; and Legends of Tomorrow on Thursdays at 8pm.