SPOILER ALERT: This article contains details of tonight’s crossover “Invasion” episode of Arrow, the series’ 100th show.

“The world around Oliver Queen has grown and so has the show,” Arrow executive producer Marc Guggenheim asserts of the significance of tonight’s 100th episode of the CW series. Kicking off the latest successful run of superheroes on the small screen with its debut in 2012, the Stephen Amell-led Arrow now finds itself not only hitting a strong benchmark for any series but also smack in the middle of the “Invasion” mega-crossover with WBTV siblings Supergirl, The Flash and DC’s Legends Of Tomorrow. The action started in the last minutes of the Maiden of Might’s November 28 show and goes until tomorrow.

Arrow, 100th Episode Party

Reflecting on Arrow’s future and past, as well as the labyrinth and rewards of the multi-show saga, Guggenheim spoke with me about how “Invasion” came together as a crossover and a stand-alone episode of the Vancouver-filmed show. The EP, who developed the show with Greg Berlanti and Andrew Kreisberg, also hinted at where things could go next for the archer and his team as Arrow heads towards next week’s Season 5 winter finale, who we may see more of from the show’s past, and what the next crossover could be like.

DEADLINE: So, right in the middle of your biggest crossover ever, you also have a sprawling Arrow episode that serves as part of the Invasion saga, a stand-alone of its own and the series’ 100th episode. What was the plan to pull that off?

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GUGGENHEIM: I mean truth be told, it is a labor of love and I think, I can’t speak for Greg or for Wendy (Mericle, EP) but I kind of approached the Arrow 100th episode from the standpoint of what if this wasn’t a TV show? What if this was the 100th issue of a comic book? As a comic book fan a milestone issue like the 100th issue that’s a really important thing. That’s a big deal and it’s a big deal for the readers. So, I was thinking a lot from my own point of view on what do I want to see? Fundamentally that was about touching as many of the characters and moments on the show that we possibly could in our 42-minute format.

DEADLINE: Going forward to the Season 5 winter finale next week, where does all this leave the show and Oliver Queen himself?

GUGGENHEIM: I think the circumstance that the Dominators placed him under in the 100th basically sort of forced him to double down on his mission, essentially to have the mental and emotional strength to pull away from the temptation of this alternate reality that the Dominators had created for him. Oliver had to re-commit to his own mission.

This whole thing has only reminded him that there’s so much more work to do, of course this being Arrow that’s usually the moment where we pull the rug out from under Oliver. I think without overstating things I think that’s very much sort of what happens in our mid-season finale next week where Oliver will re-iterate to Thea and to the other characters how re-committed he is to his mission. However, without spoiling anything, circumstances will transpire in that episode that will make that newfound drive far more difficult for him to make good on.

DEADLINE: Speaking of the Dominators, how did they become the villains of the crossover? It’s not like between Supergirl, Flash, Arrow and Legends, you didn’t already have a few bad guys hanging around?

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GUGGENHEIM: Very early on I think we all sort of collectively desired what I call an external villain. I don’t simply mean an extraterrestrial villain, I mean a villain who didn’t come from one of the shows, a brand new threat. Greg came into work one day and he’s got the ‘Invasion’ trade paperback in his hand. He’s like why don’t we do this? And I’m like why don’t we do this? That series is beloved by Andrew, Greg and myself.

DEADLINE: Was the plan always to pull all four shows into this crossover?

GUGGENHEIM: Well, basically it was always a given that after two years of doing these crossovers that we were going to do a crossover this year. I think it was also a given that we were going to include Legends along with Flash and Arrow. What wasn’t a given until the pick up happened was that Supergirl would be a part of it because Supergirl wasn’t yet on the CW – either way, we went into it very early on knowing that we were in for something very ambitious but the fans have shown us that they not only want it but appreciate it. I guess I have to include the other producers and myself in that we’re also fans. It is exciting for us while at the same time incredibly daunting.

DEADLINE: So, are you going to go ever bigger next year?

GUGGENHEIM: At the end of the day that is always up to people far higher in the food chain than myself. That said I will say that I would be very surprised if we didn’t end up doing a four-show crossover where it’s not debatable to whether it’s three shows or four shows, where it really is all four shows – and God help us.

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DEADLINE: With that proclamation in mind, now that you have two Flash/Arrow crossovers, one Supergirl/Flash team-up and the ‘Invasion’ mega-crossover under your belt, what has proven the biggest challenge to pulling these events off?

GUGGENHEIM: (Laughs) Probably the biggest challenge is actors, no matter how talented the actor is they don’t have the capability of being in more than one place than one particular time, which is very vexing. (Laughs again).

The biggest lesson we learned is how to help alleviate those scheduling issues and there’s a bunch of different little things and tricks we do. For example you’ll notice the Flash and Arrow episodes have B stories running throughout them that separate out various cast members from others. We’ve built in what we call shut down days across all three of the shows, Flash, Arrow and Legends. It doesn’t completely solve the scheduling problem but it helps make the impossible, possible and I will say judging from all the cast members I’ve spoken to and just my own experience, this year went smoother than last year despite being even more ambitious, so I definitely think we’ve learned some tricks to accomplish this.

DEADLINE: And for Arrow itself, looking at the 100th episode benchmark – how do you see the show looking back to its debut and looking forward?

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GUGGENHEIM: The world around Oliver Queen has grown and so has the show. I feel like the best way for me to really explain is the scene in Legends episode of this crossover where Oliver tells Supergirl, this is it, that Aliens is the final straw. Whenever Oliver talks about how he went into this by himself or not thinking the world was ever going to get this big, he’s really just channeling me and my reaction as a writer going Wow, the world really has gotten bigger. The way I like to think of it is that Arrow has grown into a show that’s big enough that we can do a gritty crime drama one week and an alien invasion story the next week.

DEADLINE: It does seem to have found a solid footing this season as the former more than the latter …

GUGGENHEIM: But the show now has that breadth built within it. I was watching the 100th episode with Greg in the editing room and when the spaceship chase happens, I turned to him and I’m like, how are you liking our gritty crime drama? It’s still very much that too and I think the 100th episode of Arrow is every bit as much an episode of Arrow as the previous 99 that came before.

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DEADLINE: Certainly on the level, that we see a number of familiar faces in this celebration – especially Katie Cassidy’s Laurel Lance, back from the dead in the alternative reality of tonight’s show. What was it like getting her and others from the show’s past season back for the 100th?

GUGGENHEIM: It’s terrific. One thing that’s so great about Katie is how amenable she’s been to returning to the show since Laurel died. Also, we haven’t seen the last of Katie on the show because of how big this world has gotten, just because the character is dead in one timeline is not the end of the character.

Every time we have the opportunity to have Katie back on the show it’s always a special treat. It was very important for her to be part of the 100th episode because she remains such an integral part of the show, same thing with Suzanne Thompson and Jamey Sheridan. There was a whole host of other characters who we would have loved to have gotten back if we could have worked out schedules but it’s one of those unfortunate things is that people do have other lives, other jobs and we sort of went in with our hopes of getting as many alumni back as we possibly could.

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DEADLINE: That’s what we see on one side of the camera, but what is it like on the other side of the camera for the crews and cast up in Vancouver as you guys were making the Invasion crossover and the 100th episode of Arrow that is a part of it?

GUGGENHEIM: I think when you do a season of television you go into it going I’d like the season premiere needs to be amazing. The season finale needs to be amazing. Then, of course your want the mid-season premiere and the mid-season finale to be great and we always include the crossovers in that. So, it’s a very big deal for us. It’s our big event of the year and we really give it our all, and we try to leave absolutely nothing on the field.

'Arrow' TV series 100th Episode Celebration, Vancouver, Canada - 22 Oct 2016

But to do that takes so much work on the part of so many people, the crew, the writers, the actors, everyone. It’s a job but they’re also doing it because they love the shows, they love the fans and they love these characters. So many of the people involved with these shows are fans of the characters of comics and love seeing all these people together on screen. We all get the same jolt that I think the audience gets which is oh my God, they’re all like Supergirl is sharing time with Flash and Arrow and Legends How fun is that? That to us is the whole reason we do this.