How Colton Haynes and Colin Donnell Appeared on Arrow's 100th Episode

While Chicago Med star Colin Donnell and longtime Teen Wolf and Arrow regular Colton Haynes appeared in tonight's episode of Arrow, fans likely noticed that they were...not the same kind of cameos as everyone else.

Dozens of actors came back for a few moments (or more) in Arrow's hundredth episode, but it was important to executive producers Andrew Kreisberg and Marc Guggenheim to get certain familiar faces back -- whether or not scheduling permitted it.

Kreisberg told reporters that one of his favorite parts "was getting the cast. I think that getting Jamey Sheridan, getting Susanna Thompson back, we tried desperately to get Colin Donnell and Colton Haynes. We just couldn't pull it all off."

"The show's in syndication now and I caught a couple of the early episodes on TNT. 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," Kreisberg added. "Watching those early episodes and then seeing the one hundredth and seeing Oliver have scenes with his mom and especially the scene with Thea and Moira and just remembering how important she was to the mythology and to these characters and to everything that happened. 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."

"You know it's funny that beat was not in the original break of the episode," Guggenheim said of the brief appearance of Haynes and Donnell's characters, Roy Harper and Tommy Merlyn, who appeared as holograms in the closing act of the episode. "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, I'm like, "This is the way we can get them." Props to our visual effects house, Zoic handled those shots. They did an amazing job, particularly with Colin and with Colton because we couldn't re-shoot them. They basically, we had to take them from the old episodes and roto them out and then put them into this. It was hard, obviously because they had to work with preexisting footage. Yes, they had nine hundred ninety-nine episodes to choose from, but it was a lot harder than that makes it sound. They did an incredible job, as they always do."

Arrow, The Flash, and Supergirl will cross over with DC's Legends of Tomorrow this week in "Invasion!" a three-part crossover that starts in the final moments of an episode of Supergirl and then plays through the other series. Based on the 1988 comic book event miniseries Invasion! from Keith Giffen, Bill Mantlo, Todd McFarlane, Bart Sears, and the crossover -- with its marketing title "Heroes vs. Aliens" -- will closely track the plot of that storyline: aliens will be assembled by The Dominators and descend on Earth to bring an end to the "threat" of Earth's burgeoning metahuman community.

In the comics, the Dominion were secretly hoping to build their own super-race, and as a result created a handful of new superheroes in the course of the story.

In the TV version, rather than assembling a loose alliance of dangerous alien races, the Dominators will apparently be mind-controlling the aliens that join them. That's a revelation that suggests even the heroes might be forced to take one another (or at least the alien Supergirl) on at some point in the story.

NEXT: The Flash, Arrow, Supergirl, Legends of Tomorrow Crossover Trailer / Will Joe West Die During The Invasion! Crossover? / Enter To Win An Ultra-Rare Invasion! Crossover T-Shirt / Marc Guggenheim Talks Arrow's 100th Episode / Laurel Lance's Role in Invasion! Explained / Stephen Amell Talks Arrow's 100th Episode

Supergirl airs Mondays at 8 p.m. ET/PT; The Flash on Tuesdays at the same time; Arrowon Wednesdays and DC's Legends of Tomorrow on Thursdays. All four series air on The CW. The "Invasion!" crossover will air beginning November 28.