This Is Us' Justin Hartley Teases 'Brutal' Quarrel Between Kevin and Randall: 'It's a Tough One'

Though Rebecca (Mandy Moore) was finally able to see John Singer Sargent’s “Portrait of Madame X” painting at The Met again, her sons Kevin (Justin Hartley) and Randall (Sterling K. Brown) did not have as great of a trip to the Big Apple.

In Tuesday’s episode, titled “New York, New York, New York,” the Big Three (mostly the brothers) contemplated placing their ailing mother in a nine-month St. Louis clinical trial to aid her memory loss. Randall spent hours researching the new testing without his siblings and presented the idea to them, confident that they would all agree. Though Kate was on board, Kevin was hesitant with the thought of having Rebecca spend most of the year away from her family.

As timelines jumped to the past, it was evident how much Kevin and Randall had different opinions on what was best for their mother. From teen Kevin attempting to set up Rebecca with his acting teacher a year after Jack (Milo Ventimiglia)’s death to Randall constantly worrying about her safety, the Pearson brothers have had a history of fights.

However, Hartley tells PEOPLE their biggest quarrel could be just around the corner, especially since Rebecca was adamant about “spending her good years” before “running out of time,” deciding herself to not partake in the trial.

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“These two are constantly bickering back and forth. They’ve been arguing their whole lives, and they’ll continue to argue their whole lives. But there is this one thing that happens where it’s a stalemate,” the actor teases. “I’m taking a hard line and you’re taking a hard line. I’m not budging and he’s certainly not budging.”

Their sibling rivalry came to blows in New York when Randall blamed Kevin for not taking the chance “to save Mom” with the trial and revealed he’s haunted by the daily shoulda-woulda-coulda about the house fire that led to Jack’s death.

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Hartley breaks down the forthcoming fight between Kevin and Randall, which may be the catalyst for their rift that was foreshadowed earlier this season.

“It turns into one of these things where it’s like, ‘Look if this is the way it’s going to be, you’re not going to be in my life.’ It’s brutal,” the star says. “They come to a point where it’s like, ‘These are the choices I make and this is how I live my life.’ And the other brother is like, ‘Well Jesus man, I don’t what to tell you. I can’t have that in my life.’ ”

Hartley adds, “It’s a tough one. Because I like it when they’re getting along as well, you want them to get along.”

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This Is Us showrunner Isaac Aptaker previously told reporters that fans will see Kevin and Randall’s “tear apart” by the end of season 4.

“By their 40th birthday, by the start of next season, they had this falling out and are not on speaking terms. They were beautifully there for each other and had a pretty incredible brotherly relationship so something big is coming in this next run before the end of this season that will tear these two apart,” he said.

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