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'This Is Us': Justin Hartley Warns Kevin & Randall's Huge Fight Changes Their Relationship Forever (Exclusive)

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Whatever tears Kevin and Randall apart on This Is Us is so massive that by the time their 40th birthdays roll around, they've stopped speaking to each other. While there haven't been any hints to any cracks -- even small ones -- forming in their relationship (they've actually been leaning on each other quite a lot for support), co-showrunner Isaac Aptaker suggested that "something big is coming." But what could be so "big" that would put the two brothers at odds that they can't even be in the same room together? 

Justin Hartley contextualized just how major this game-changing moment for Kevin and Randall is, saying that the brothers' upcoming blowup will effectively change their relationship for good. And gulp, you should be very, very worried.

"It's big. No, it's big, big," Hartley tells ET. "It's more than a misunderstanding. I think it's a philosophy. It's like, 'This is how you handle your life and this is how you deal with things and this is what you think. I go about my business a different way,' and they just can't [see eye to eye]..."

While Kevin and Randall have experienced their share of ups and downs over the past four seasons, Hartley says this is on another level. 

"This one's for real," he says. "This is not like we're going to argue about something. This is two men, two brothers each taking a stand that suggests that if they don't bend, the other one will not be in their life. I mean, it is a big deal. It just gives them... it's a big deal."

Hartley shared that they're finally getting into the meat of Kevin and Randall's conflict in the coming days, meaning it will likely be a season-ending stunner.

"We're still shooting that. We're still telling that story," the 43-year-old actor reveals. "[This] week we start to get into it."

And though Hartley wouldn't disclose details, he expressed excitement over digging deeper into that aspect of Kevin and Randall's complicated dynamic. 

"It'll be interesting. That'll be fun though. I mean, I love that man. I love working with him," he says of working alongside Sterling K. Brown. "Look, we have fun. We laugh like crazy people when we work together. So even when we're 'fighting' on the show, we have a great time together. I have all the respect in the world for him and we're great friends. It'll be fun mixing it up with him."

In the fall finale, it was revealed that Kevin and Randall were no longer on speaking terms when their 40th birthdays rolled around, a stark contrast from their supportive brotherly bond in recent episodes. 

"It's definitely something that's coming out in the near future run of episodes. By the 40th birthday, by the start of next season, they've had this falling out. They're not on speaking terms," Aptaker said in January. "In this episode [that aired Jan. 21], they're so beautifully there for each other and they have this pretty incredible brotherly relationship. So something big is coming in this next run before the end of our season that will tear these two apart."

This Is Us returns Tuesday, Feb. 11 at 9 p.m. ET/PT on NBC.

For more on This Is Us season four, watch ET's exclusive interview with Chrissy Metz below.

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