On the CW's The Flash, you either die a hero, or you live long enough to see yourself become the villain (more than once) and your archenemy become the hero.

At the end of "Armageddon Part 3," Barry Allen (Grant Gustin) sped to the year 2031 to prove he doesn't cause the titular apocalyptic event. When he arrived, though, he was horrified to discover that his nemesis Reverse Flash, a.k.a. Eobard Thawne (Tom Cavanagh), was in a relationship with his wife, Iris West-Allen (Candice Patton). The future shock will continue in "Part 4," airing Tuesday, because it turns out Barry is the murderous Reverse Flash in this alternate timeline and Thawne is Central City's heroic scarlet speedster. What is going on, and how did this happen?

"I'm going to leave that to smarter people than me to talk about," Cavanagh tells EW, trying to avoid spoilers. "I think the larger point, which doesn't give things away, is when you've put in the amount of time that you put in when you're telling a Batman-Joker story or Lex Luthor-Superman story or Flash-Reverse Flash story, you've earned, in many ways, the big face-offs and twists that renew the rivalry. This is an instance when I felt like the writers' room did a tremendous job of organically working up to the clash of these two people."

The Flash
Grant Gustin as Reverse Flash and Tom Cavanagh as the Flash on 'The Flash'
| Credit: Katie Yu/The CW

The last time we saw Cavanagh's villain was in the season 7 finale. After helping Barry defeat Godspeed, Thawne tried and failed to kill Barry because his former pupil was finally faster than him. So what's driving him in this alternate timeline where he's the Flash?

"If you've had something really consume you and you think, 'I wish this thing didn't consume me, but I can't help it,' that's some insight into what we're doing in 'Armageddon,'" Cavanagh teases. In terms of whether Thawne genuinely cares for Iris or if that relationship is simply part of his long-standing rivalry with Barry, the actor offers, "If you layer it on as just one thing, you might be missing some opportunity there. What's nice is you know that Eobard Thawne, as written [on the show] and in the 80-plus years [of comics] canon, is somebody who doesn't really leave things to chance. As an actor who likes to explore as many different colors and layers I can put in there, I think the idea that those two people come together on screen finally for the first time in forever, that's a very large moment in the show, and one that was a delight to play."

Cavanagh, who departed the Arrowverse drama last season, was particularly excited about switching costumes with Gustin. "For both of us, when you get into season 40 of a television show, sometimes the cool-factor moments can be a little elusive, and this was definitely a thrill," he says, praising costume designer Kate Main and the artisans who craft the suits. "In the age of social media, you're going to be immediately judged. So there's a challenge there. I felt like everybody rose to it. For me personally, it was a real privilege to put on the Flash suit and play the Flash."

The Flash
Tom Cavanagh as the Flash, Kayla Compton as Allegra Garcia, Javicia Leslie as Ryan Wilder/Batwoman, Brandon McKnight as Chester P. Runk, Osric Chau as Ryan Choi and Danielle Nicolet as Cecile Horton on 'The Flash'
| Credit: Katie Yu/The CW

The Flash airs Tuesdays at 8 p.m. ET/PT on the CW.

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