'This Is Us' season 1 news: Mandy Moore on how Rebecca and Jack's relationship deteriorates

The third episode of "This Is Us" season 1 revealed a major twist — a life-altering secret Rebecca (Mandy Moore) had been keeping from her son Randall (Sterling K. Brown) since he was little.

In "Kyle," the NBC dramedy returned to the day Rebecca and Jack (Milo Ventimiglia) brought their babies home from the hospital and began their new life as young parents.

Meanwhile, almost four decades into the future, Kate (Chrissy Metz) and Kevin (Justin Hartley) were about to move to New York City to begin chasing Kevin's dream of becoming a theater actor. Later, Randall told his mother that he had finally tracked down William (Ron Cephas Jones), his biological father.

But while the present-day Rebecca was seen seemingly meeting William for the first time, the episode revealed that the two had already met. Shortly after her twins were born and she decided with Jack to adopt Randall (who was almost named Kyle), Rebecca had seen William outside the hospital. She later tracked him down and made the decision to keep the former drug addict away from Randall.

In an interview with The Hollywood Reporter, Moore explained Rebecca's choice as the "best possible decision" that the young mother could make for her family at the time. She also said that the guilt and fear that Randall will learn about his father had been weighing on the woman for years, especially as Randall appears to be the child she is closest to.

The actress also spoke about how Rebecca and Jack's relationship will continue to be tested, saying, "What we saw in the second episode continues."

According to Moore, Rebecca's feeling that she is not getting enough emotional support from Jack will "definitely take a toll on their relationship and on the relationship that they have with their children."

"She feels resentment that she sacrificed her career, which we'll find out more about," Moore added.

Speaking with Vulture, the "This Is Us" star addressed the revelation that Rebecca and Jack will not be together — and that Rebecca will be with Miguel (Jon Huertas), Jack's best friend — in the future.

"I'm a bit heartbroken by the fact they don't end up together, but I'm also really curious to see how and where the story takes them," she said.

"People hate Miguel," she added. "I'm like, Just wait, wait. There's so much story to unfold. I don't know if people will necessarily have that reaction once things are revealed."

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