SPOILER ALERT: Do not read if you have not yet watched “Songbird Road, Part One,” the 11th episode of the third season of “This Is Us.”

“This Is Us” patriarch Jack Pearson (Milo Ventimiglia) may have kept a big secret from his family — that his brother Nicky (Michael Angarano) did not actually die in the Vietnam War — but it turns out Nicky kept quite a secret from him, as well.

The boat explosion in Vietnam that caused Jack to dive in the water after his brother left Nicky unscathed physically, but because it claimed the life of a local boy, he was quite scarred mentally. Jack didn’t know that Nicky had taken the boy fishing for fun — or that the boy reached for one of the grenades Nicky was tossing overboard to kill and catch the fish, causing the grenade to land in the boat and take his life after Nicky jumped overboard. In the moment, Jack just saw reckless behavior and yelled at his brother that he was “done” trying to help him. Years later Nicky would mail him postcards but never gave him the fuller picture of the event.

“Nicky made attempts to communicate with his brother, but he also, I think, understood how upset Jack was and he probably just never pushed the issue,” Ventimiglia tells Variety.

But Nicky wasn’t alone in that. Jack never wanted to talk about what happened either — so much so that he cut his baby brother out of his life.

“There have been very few things that Jack has done that I haven’t agreed with, but not talking to his brother was definitely one that upset me,” Ventimiglia says.

Here, Ventimiglia talks with Variety about filming that pivotal Vietnam sequence, working with Angarano as slightly older versions of their characters, and how it feels to continue to peel back layers on the character three seasons in.

“This Is Us” has been teasing that explosion and Jack’s dive into the water for quite some time now. How did it feel to actually film it?

Most of Vietnam was filmed here in Los Angeles, just up north of the city, and we shot that bit in November, here in LA. It was really, really cold. And there was a break — we had an episode in between where Jack jumps in the water and then where we actually filmed the stuff of Angarano and I in the water. It was one of those harder days — because there was a lot that was spoken but not necessarily heard. What I’ve really enjoyed with Jack and Nicky, they’re Nicky’s stories that also happened to Jack; they’re Nicky’s stories that are sort of seen through Jack’s lens at times, but we really leaned into making Nicky’s experience through being drafted, through having a hard time in the war, through doing something so horrible that his brother would never forgive him. There was a lot that was said when we were filming that that didn’t really make the cut, but also the way it was produced in post muted out the sound and brought everything down so you’re in Nicky’s head because of course it’s his story to tell when he’s older.

Do the added elements of having to focus on stunts or being distracted by cold weather get you out of your head for some of the more emotional moments?

Yeah, and we have some of the best writers, who give us dialogue that sticks in your brain so you don’t have to think about it. And by not thinking about it, you can just experience it. I can say 100% Mike and I, that day, he had made a mistake, and I was cutting him loose. It was one of those days that, yes, it was difficult, but coming out the other end, my hat’s off to Mike for what he’s done with the character, and this episode, I think, is really special for him and for all of us.

On the other side of the spectrum, how emotional was it to film the scene with young Kevin (Parker Bates) where Jack is imparting a lesson about doing better than his father?

When I watch the episode or read further and see that Kevin does do better than his father did, it’s strange [but] I feel a little sense of pride that he listened. It’s bizarre. I definitely experience everything as Jack but also watch it as Milo, the audience, so it’s a mixture of pride and resolve, and it’s good to see growth and people learning — whether Kevin remembered that moment or we were just showing it, I think who Kevin has become because of his own conviction and the strength that his dad and his mom have put into him, he’s a good man.

How did you justify for yourself why Jack would go so long without talking to his brother and telling his family Nicky was dead?

I understand the value in the story. I understand the value in having Jack be a guy who made that right turn as opposed to the left to go back in the past and pick someone up. It’s compelling storytelling. And things don’t always end up nice and easy. Things aren’t always perfect. … I do understand men of that era — men coming out of war — how they don’t want to revisit, they don’t want to go back to where they were in the past. So I can kind of understand Jack’s decision to move forward in his own life, but also I just always wonder if Jack only has enough capacity and room to be responsible for a small group. When he was younger he felt very responsible for his brother but realized his brother was making his own decisions and choices, and as Jack got older, he was like, “I can’t plug that in.” Time goes on and the valley widens.

He did hesitate at that literal fork in the road, though. Do you think if Nicky had kept sending him postcards things could have been different?

He is a man who lives in the positive, and I don’t know what it would be like to really have him crack that open, and sadly we’ll never know. I feel like Jack, had he made it past his 50s and was around in the present day and seeing his wife and his kids’ struggles, he probably would have softened up a bit and decided he might be willing to open up those past wounds and let them heal properly.

How did you and Michael find yourselves working differently when the Pearson brothers saw themselves post-war, slightly older and with that extra layer of emotional history?

It was the first time Mike had worked in a full prosthetic beard, and I had grown pretty accustomed to them, so when he and I were laughing about something on set, we couldn’t laugh too loud because we didn’t want to break up the work of Zoe Hay, our makeup artist. But it [also] felt uncomfortable. We had spent so much time in the younger years of our characters that to jump forward and see different sides of them was uncomfortable. Jack of that age, what I know is he’s there with his kids, he’s there’s with his wife — he’s really all about them. So to be with his brother was a different experience.

Was it ever on the table that Jack would let Rebecca in on the truth, even if not his whole family?

For Jack the war does not represent anything heroic, anything good for him. He’s ashamed of what went on and he doesn’t want to bring that into his life with Rebecca, with the family. He’s even said it…when she was trying to get him to talk about the war. The sad truth is that he never was going to tell her, and he never got to tell her. But it opens up the door for so much in Rebecca in the present day and the Big Three to explore. It’s beyond Jack.

Now that you’ve been peeling back layers on Jack for three seasons, is there anything the writers could say about him that would throw you for a loop?

I don’t know that the writers would do that to me, or to us. There will always be things to uncover about Jack, but I don’t see them changing course for who he is. We all know that Jack is a good man and he’s a flawed man. What is good is and what his flaws are, we’re still uncovering, but we do know a lot about both of them.

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