Katherine McNamara on 'Crisis on Infinite Earths' and Taking Over the Green Arrow Mantle (Exclusive)

stephen amell kat mcnamara arrow crisis on infinite earths crossover
Katie Yu/The CW

The actress spoke with ET about the 'sacrifices' to come in the final episodes of the crossover event.

The "Crisis on Infinite Earths" crossover is finally here, and it is threatening to change everything in the Arrowverse.

The first two parts of The CW's five-episode event -- which brings together DC heroes from Arrow, The Flash, Legends of Tomorrow, Supergirl, Batwoman and Black Lightning -- have already dealt some devastating blows to the team, and things are only going to get more difficult as they strive to save the multiverse from a powerfully destructive supervillain, the Anti-Monitor.

The first casualty of the ongoing battle was a brutal one, as the Green Arrow, Oliver Queen (Stephen Amell), sacrificed himself as he held off the attack long enough for civilians to evacuate. The heroic archer succumbed to his injuries while surrounded by his team of friends, though the all-seeing Monitor ominously warned, "It was not supposed to be like this. This is not his ending I foresaw." 

In the second part of "Crisis," some of the team, along with our old friend, Constantine (Matt Ryan), traveled to yet another Earth to find a Lazarus Pit to try and resurrect their fallen friend -- but as previous attempts have shown, not all of him came back unscathed. The loss of Oliver's soul left something else in his place, something far from the caring and courageous hero they knew.

The loss was especially painful for Oliver's daughter, Mia (Katherine McNamara), who was finally getting to know her father -- Oliver was taken away by The Monitor when she was just a baby -- after a twist in the timeline brought them together as adults.

"That's the crux of Mia's growth in this entire season is that, you know, she's had this massive loss, this hole in her life in the entire time she's been alive. She hasn't known her father and she's been kind of yearning for that connection," McNamara explained to ET during a phone interview last week. "And then obviously, when she was confronted with it, it was a very, a much more difficult journey than I think either of them expected, coming to terms with all of the abandonment and loss and things that she had been harboring and trying to push aside, forcing them to come to the surface when he's standing right there in front of her."

"Once that's finally been reconciled and she understands him so much more and has come to terms with opening up that connection, that relationship and that love is there that they've both found for each other, that they've been yearning for their entire lives, it's all just kind of taken away in an instant -- and far too soon. And coming to terms with that is one of the most difficult things that Mia has ever had to do. It definitely changes her, for better or for worse. It changes her path forever."

A recurring theme throughout Arrow's eight-season run as The CW's flagship superhero show is the concept of legacy, and Oliver -- foreseeing his own demise as The Monitor prophesied -- made certain of his before he died, passing down the Green Arrow mantle to his daughter, despite her insistence that only he should wear the signature hood.

"I think it's really overwhelming for her," McNamara noted. "She's certainly never thought of herself as a hero or being worthy of taking on that mantle, given her checkered past and her relationship with the vigilantes, and with this team... Creating that element of that redemption story that he has, she sort of mirrors that in a way."

"And it's something that she understands, you know, much in the same way that, in enjoying the show, I understand the legacy that Steven and/or Oliver had built... It becomes a really interesting turn in the character, and having someone so different and yet so similar take on this mantle."

Part of inheriting that legacy meant donning her own Arrow suit, which was a major moment for both McNamara and her character. Ironically, her first scene in the suit was shooting Oliver's emotional goodbyes as he died -- meaning she had to tamp down her excitement over her new gear -- but the rest of the experience has been "a blast."

"It was such a fascinating process because the folks that design and create these suits are such masters, they've done this so many times now and created so many different characters, so many different suits," recalled the actress, who is set to continue on as the new Green Arrow in an upcoming spinoff series. "It was a treat just to watch them work and sort of create this next generation for this character. But on top of that, it was such a blast getting to be on set with all the other superheroes and their suits and getting to have my own suit and run around."

And the team-up is far from over. "Crisis" continues on Tuesday with The Flash's installment, before returning in the new year with a two-part finale on Tuesday, Jan 14. As the team mourns one of their own, there's still the matter of saving the multiverse -- a "massive," life-changing mission for Mia, who has until this point been mostly confined to her hometown of Star City.

Katie Yu/The CW

"Now suddenly she's on a spaceship with people with superpowers, and aliens, and dealing with all of these other things," McNamara previewed. "Suddenly being surrounded by strangers -- people that know her, but she doesn't know them -- in one of the most traumatic times of her life, it really kind of puts her out of her element once again, and forces her to grow in a big way."

As the crossover newcomer this year, it was a feeling that the actress could relate to -- but she said the experience was one that she'll never forget.

"We have so many of these crazy moments, in the job that we do as actors, that occasionally we forget how special these are, but sometimes it just hits you in the face," she said with a laugh. "When you're getting on set and there's, you know, Superman and the Flash and a few other supers, and you're standing there looking around in a supersuit of your own going, wow, this is kind of amazing. These are the moments that will stay with you for the rest of your life."

Check out the remainder of The CW's "Crisis on Infinite Earths" schedule below:

Part 3: The Flash – Tuesday, Dec. 10, 2019 (8:00-9:00 p.m. ET/PT)

Part 4: Arrow – Tuesday, Jan. 14, 2020 (8:00-9:00 p.m. ET/PT)

Part 5: DC’s Legends of Tomorrow – Tuesday, Jan. 14, 2020 (9:00-10:00 p.m. ET/PT)