LOS ANGELES >> “This Is Us” star Milo Ventimiglia was between scenes on the set in Vietnam, the setting for a critical flashback story about his character Jack’s wartime tragedy.

Then modern-day reality merged with TV fiction about a decades-old conflict.

“I’m there in full battle dress, my rifle slung, my hat on, looking very much the 1970 U.S. soldier, and an older gentleman going past us said something in Vietnamese, did a double-take on me and then just kept going,” Ventimiglia said of last month’s encounter outside Ho Chi Minh City.

The Vietnamese actors he was working with suddenly burst out laughing, then filled him in.

“What the hell? What the hell is a U.S. soldier doing back here?” the man exclaimed as he zipped by on a motor scooter, Ventimiglia was told.

It was an isolated blip in an otherwise smooth week of shooting in Vietnam, with the scenes to be scattered throughout “This Is Us” episodes as the series gradually unravels — in its trademark style — a key chapter of Jack’s life.

The NBC drama, which toggles back and forth in time between moments big and small for its characters, is zeroing in on what Jack endured during the failed U.S. war to defeat communist North Vietnam: The death of his brother, Nicky, played by Michael Angarano.

That’s no spoiler, as “This Is Us” viewers know. But unlike the prolonged, angst-building revelation of how family man Jack died, series creator Dan Fogelman said the circumstances of Nicky’s fate will be known by this season’s end.

He is well aware of the audience’s investment in Nicky, he said, with his own anxious friends and family serving as a barometer. Fogelman credits the actors playing Nicky as a child and adult (Donnie Masihi, Angarano) and the bond between Jack and the younger brother he is driven to protect.

“It’s definitely something that’s a big part of our season and that we’ll get all the answers for by the end of the season,” he said. Does that include learning the identity of the woman shown alongside Jack in an intriguing wartime photo?

“Yes, exactly,” Fogelman said, a reply at once comforting and opaque. He’d already made the point that providing answers on “This Is Us” often means raising more questions.

There will pain along the way, said Ventimiglia, who plays the patriarch of the Pearson family that includes wife Rebecca (Mandy Moore) and offspring Randall (Sterling K. Brown), Kate (Chrissy Metz) and Kevin (Justin Hartley). Even he was more deeply affected than usual by what Jack is facing, the actor said.

“When I watch the show, I’ve cried for Rebecca, I’ve cried for the kids. I never cry for Jack,” Ventimiglia said, but that changed after he previewed the episode airing Tuesday. “It was really heartbreaking to see him go through what he went through.”

As with all “This Is Us” joys or misfortunes, there’s a ripple effect for generations ahead. This week’s episode has Kevin and girlfriend Zoe (Melanie Liburd) arriving in Vietnam, with flashbacks uncovering more of Jack’s ordeal there. (There’s also a Jack-Rebecca road trip to Los Angeles, circa the couple’s early days together.)

“We’re showing the audience Jack’s experiences in Vietnam and we’re also showing the impact that it’s had even beyond that,” Ventimiglia said. “His son Kevin is digging in deeper, trying to understand what his father went through to connect to him,” with other siblings involved as well.

The Vietnam story line, which earned a rare hour of its own last month, is woven in the narrative as the season progresses. But Fogelman said its importance, and the lack of a military veteran on the show’s writing staff, warranted bringing in Vietnam veteran and acclaimed writer Tim O’Brien (“The Things They Carried”) as a consultant.

O’Brien said he’s been impressed by the show’s painstaking approach and its artistry.

“They’re just totally committed to making getting everything right,” he said, in contrast to many other Vietnam dramas. “But even more important to me was the quality of the show. … They seem so committed to making a beautiful show that touches on human pain and struggle and human joy.”