Mandy Moore Talks Filming One of Her Hardest Scenes for This Is Us — but It's Not What Fans Think!

Mandy Moore had countless emotional scenes to film throughout This Is Us's six-year run, but one stands out among the rest.

While speaking at the series finale screening and panel on Sunday, Moore opened up about the challenges of playing matriarch Rebecca Pearson and how scenes in the penultimate episode ended up being among her most difficult to film — even though she didn't have any lines.

"One of the harder parts was the very end [of 'The Train'] episode," explained the actress, 38. "I remember thinking, 'Okay, I've gone through four hours of makeup and I get to just lie in bed and listen to my friends say these beautiful things about this character.' I was like, 'Ugh, I can't wait, what a great day!'"

But that all changed when Susan Kelechi Watson, who plays Beth Pearson, walked through the door to film her goodbye speech to her on-screen mother-in-law.

"Susan walks in first, and delivers that whopper of a monologue. Thank God the camera was on her because I just started crying with my eyes closed," Moore recalled. "I had tears streaming down my face. I'm like, 'This is not okay! This is not my scene, this is Susan's moment! I'm not supposed to be crying, I'm not really present, I'm not really here.'"

"I would say that was oddly one of the more challenging parts, having to put myself in a completely different frame of mind," Moore continued. "I didn't get to listen to anything. I did get to see everyone's work in the episode, obviously, but I was like, 'I can't listen.' I was gonna cry all day!"

Ron Batzdorff/NBC Sterling K. Brown as Randall and Mandy Moore as Rebecca on This Is Us

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In response to her story, Moore's costar Milo Ventimiglia, who played her onscreen husband Jack Pearson, couldn't help but poke fun at his own experience dying on the series.

"Try being dead in a hospital," he quipped. "When you had that meltdown [in season 2]? Yeah same, now you know."

"It sucks!" Moore responded, to which Ventimiglia jokingly added, "It's horrible. F---ing' worst."

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Ron Batzdorff/NBC Justin Hartley as Kevin, Sterling K. Brown as Randall, Mandy Moore as Rebecca in This Is Us

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During Sunday's panel, Moore also spoke about portraying a character who lives with Alzheimer's, calling it "the most terrifying and daunting aspect of anything having to do with the show."

"But I also trusted Dan [Fogelman] and our writers that they would make sure it was done with such consideration and grace and precision in the way that I think we have approached so many challenging subjects on the show," she explained. "This is not part of my story, but I did recognize it is millions and millions of peoples' across the country and around the world, and I felt an incredible sense of responsibility to make sure we got it right."

"I knew the writers would obviously do their own due diligence, but I wanted to make sure I did my own, and so, on top of the research for this season, I was really, really lucky to be able to talk with a neurologist who had conferred with the writers, just to ask questions," she continued, adding that while not everything they spoke about was incorporated into the series, it was "important [for her] to have that framework and understand the frame of mind."

Ron Batzdorff/NBC Mandy Moore and Sterling K. Brown on This Is Us

This isn't the first time that Moore has opened up about the emotional toll that This Is Us's "The Train" episode took on her.

In April, the actress revealed that she threw up after reading the penultimate episode's script. "It was so beautiful and upsetting that that was my physical reaction," she said.

Moore's costar Chrissy Metz added that she had a physical response to reading the episode as well. "I couldn't breathe. I couldn't catch my breath," she said.

This Is Us premiered on NBC in September 2016 and has been on the air for six seasons. In addition to Moore, Ventimiglia, Metz and Watson, the series also stars Sterling K. Brown, Justin Hartley, Chris Sullivan and Jon Huertas.

The series finale of This Is Us airs Tuesday at 9 p.m. ET on NBC.