Beware! Major spoilers for Echoes ahead. 

Michelle Monaghan didn’t quite understand the scope of her role in Echoes when she started reading the first script. “I was [originally] thinking, ‘Oh no, this isn’t really twins. This is just one person, let’s be real,’” Monaghan tells Tudum ahead of the show’s Aug. 19 premiere. 

But then she reached the premiere’s shocking final twist: Identical twins Gina and Leni have been secretly switching lives for years. Monaghan gasped as she realized she’d  have to play two separate women with different motivations, personalities and styles, whose lives are so intertwined that even they can’t tell who’s who anymore. Then she picked up the next script; challenge accepted. 

“This is the greatest, biggest, most ambitious endeavor I could take on,” says Monaghan, whose past credits include thrillers like Kiss Kiss Bang Bang and Gone Baby Gone, as well as ambitious TV dramas True Detective and Messiah. “I've been fortunate to be working in all the genres over the span of the last 20 years, but nothing prepared me to take on something of this caliber. It required a lot. And I had so much fun doing it.”

Created by Vanessa Gazy and helmed by 13 Reasons Why creator Brian Yorkey, Imogen Banks (The Beautiful Lie), and Quinton Peeples (Runaways), the limited series begins with a crisis: Gina gets a call one evening to learn that her sister Leni has gone missing. The two sisters are close despite living very different lives. Gina is a successful Los Angelesauthor who lives in a glass mansion with her hunky therapist husband Charlie Davenport (Daniel Sunjata), while Leni is raising a child and running the family horse farm in Virginia with her own hunky husband Jack (Matt Bomer). But when Gina arrives on the scene to help with the search, the audience learns that the ties that bind the two sisters are more complex than just blood. Every year on their birthday, the two women trade lives, fooling everyone, even their husbands — another selling point for Monaghan, who points out, “Why wouldn’t you trade lives [for] Matt Bomer and Daniel Sunjata?”

In fact, the woman we first get to know as Gina isn’t even Gina. She’s Leni, caught in the middle of a year posing as Gina. And as it turns out, her sister has been hiding a few secrets of her own this whole time. 

Monaghan was instrumental in helping to shape the two women, infusing each of them with separate characteristics and individual personalities. “Gina's very grounded and Leni is very fleeting,” she says. “[Leni’s] got a lot of that Southern charm and it's hard to pin that one down. She's always two, three steps ahead. Gina's just really more of the quiet giant, really the observer.” 

Those details are revealing, but Monaghan didn’t stop there. To really get a handle on her characters, she went all the way back to the beginning. “I started to develop them really from the womb, Leni being [born] first and then Gina second. Then [I] really used the baseline of that foundational trauma of them losing their mom, their very different memories of it and how that memory informed each of them emotionally, physically and energetically. I built Leni from top to bottom, then I built Gina from there,” says Monaghan.

Okay, but what are the logistics of filming a scene with yourself? “That was the other daunting thing,” she says. “I was like, ‘Okay, so now what? I know who they are. Now what do I do when they're in the same scene?’"

As anyone who grew up on Who Wants to Be a Millionaire knows, the answer is always to phone a friend. Monaghan would shoot her scenes first as one sister, then the other, with a fellow actor standing in as the opposite part. If Monaghan was playing Leni, a stand-in would read Gina’s lines. “Then I would go into hair and makeup for about an hour and switch over into Gina.”

If that sounds exhausting, it’s because it was. “We would do that sometimes a couple of times a day,” she says. “There were many nights at 3:00 a.m. where [they’d be] doing my hair, applying my makeup, putting in the earrings, the whole thing.” 

Jackson Lee Davis/Netflix

The most unexpected challenge was having to learn both sides of a stunt choreography — one episode has the two sisters physically fighting each other, which Monaghan can only describe as “intense.” And then there’s a pivotal scene in Episode 6, in which Monaghan transforms from Gina into Leni in real time while being interrogated by Sheriff Floss (Karen Robinson), who suspects the two sisters may be involved in something criminal. Once again, Monaghan says she leaned on her co-stars for support. “​​Each co-star, each character treated Gina or Leni differently,” she says. “That gave me so much to use.” 

Also essential were her carefully marked-up copies of each script, which Monaghan covered with Post-it notes to herself and highlighted details that would hammer the performance home. “That was really what allowed me to keep things as straight as I could,” she says. 

For many actors, playing one character is all-consuming. You’ve probably heard the stories of performers who get so wrapped up in their role that they refuse to break even in between takes. Try two roles at once! Monaghan says it took her a couple of months to truly come down from the adrenaline that fueled her performances and finally make space for herself. “You're living, eating it and breathing it, and it really does serve the material at the time,” she says. “But I came home and I crashed. It was right around Christmastime, so I got to spend quality time with my children and feel like a mom again.”

Ultimately, Monaghan looks back at the five months she spent filming Echoes as some of the most deeply rewarding work of her career. “Creatively, I'm always like, ‘I don't want to be put in a box,’” she says. “I want to continue to explore things that I haven't done. I want to continue to be challenged. I want to learn. And this was definitely one of those learning opportunities that I didn't know if I could actually pull off. I hope I just get to continue to do more things like this.”

Still, nothing can really prepare you for the experience of watching yourself opposite your mirror image on-screen. Monaghan’s verdict? “I’m so proud.”

Echoes Trailer