'This Is Us' : Mandy Moore on Rebecca's Health and Her 'Meaningful Song' at Kate and Phillip's Wedding

The Pearson family has officially entered a new chapter of life on This Is Us.

During Tuesday's episode, titled "The Day of the Wedding," viewers saw Kate (Chrissy Metz) finally get married to her music school boss, Phillip (Chris Geere) after her divorce from Toby (Chris Sullivan). However, the happy moment also saw a challenging development for the Pearsons.

Ahead of the ceremony, Kate, Randall (Sterling K. Brown) and Kevin (Justin Hartley) began to witness a noticeable health decline in Rebecca (Mandy Moore), who was planning a performance at the wedding despite her ongoing dementia. This presented itself in Rebecca mentioning Jack (Milo Ventimiglia) multiple times throughout the day as if he was still alive and calling Kevin by his father's name.

Though Randall attempted to address her health with Miguel (Jon Huertas), he was unsuccessful in his efforts. An emotional Miguel later revealed how life is "moving so fast that I have whiplash," and his sentiments eventually inspired part of Randall's speech at the reception.

Additionally, the wedding day saw Beth (Susan Kelechi Watson) and Madison (Caitlin Thompson) attempting to find out which woman Kevin spent the night with –– Cassidy, Sophie or the wedding singer.

Elsewhere in the episode, viewers were taken back to the mid-1980s, where Rebecca struggled with the predictability of her daily routine and decided to cut her hair like Princess Diana. After becoming regretful over the decision, Jack shaved off his beard in solidarity and the two went out on a date, where Rebecca spontaneously decided to sing at the piano — a moment that ran parallel to her present-day wedding performance.

Below, Moore tells PEOPLE all about the events of the wedding day and what Rebecca's declining health could mean for her family in the remaining five episodes of This Is Us.

PEOPLE: What was it like filming this important part of the Pearson story? Was it hard to film knowing it was giving viewers such a clear view into Rebecca's health decline?

MANDY MOORE: This is the first time we've jumped and lived in this time period with the whole family for a good amount of time. It was emotional to really put into context, just how much of their lives have changed, including Rebecca's declining health. It's not a fun space to live in as an actor, knowing this is the reality for millions of men and women and their loved ones in throes of the reality of this disease.

Rebecca's wedding performance brought tears to everyone in the room. What did singing that song mean for your character? How special was it that Rebecca rehearsed it with Phillip for a year beforehand?

My husband [Taylor Goldsmith] and our show's composer Sidd Khosla wrote the song — as they have with several songs over the course of the series — which made it even more meaningful. I conferred with a neurologist who has helped me and the writers make sure we are as authentic as possible with the details. I discovered that, oftentimes, if someone suffering from dementia has done something repetitively in the past, as Rebecca has with playing music, then their cognitive reserve makes that same skill available to them, even years into a diagnosis. I loved that Rebecca was able to rise to the occasion and sing such a meaningful song at her daughter's wedding.

My mouth dropped when I realized the song was the This Is Us theme. Was that the intention?

It's my understanding that Dan [Fogelman] always had the idea for the song to come around in a more fully realized form.

How do you hope fans who are directly affected by dementia and Alzheimer's will respond to this episode?

I hope that this show engenders more of a dialogue around dementia and Alzheimer's in general. It's a terrifying diagnosis and helping to humanize and destigmatize what it means to find yourself or a loved one in this particular battle is a big deal. We need more compassion, understanding and funding for research. The reality is this disease is going to continue affecting millions of us around this country and the globe, and I hope our show can be a beacon of community and model one family's response to it, especially for those who know this fight all too well.

Why was it important to show a flashback of Rebecca in her daily routine and her hesitance to break it? What do you believe those activities accomplished for Rebecca at that point in time?

Being a mom equates to being a creature of habit sometimes and I think it's heartbreaking to watch a younger version of herself yearning for spontaneity, only to be robbed of that choice later in life.

There was a hint to Miguel's own health issues. What can you reveal about that and what does it say about his character's love for Rebecca?

If anything, I think continuing to show Miguel's unending dedication as a partner further highlights how easy it is for caregivers to put their own well-being on the back burner, and that can — and will — slowly catch up to you.

Rebecca also said she believed Kevin would work things out with whom viewers are led to believe is Sophie. Why does she feel so strongly about them reuniting despite them taking different life paths?

I think Rebecca's condition makes it so she's stuck in the past with a younger version of Kevin and Sophie, but her belief in their strength their love and as a couple transcends time.

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What do you think Randall's heartfelt speech at the wedding says about his acceptance of his mom getting older? Do you feel like that speech was especially relevant with the end of This Is Us nearing?

So much of this season and life, in general, is about accepting the changes and endings and new beginnings. Memories are the glue of life. It's especially poignant because of where we find ourselves with the closing of this chapter but it's really what the show has been echoing all along.

This Is Us airs Tuesdays at 9 p.m. ET on NBC.