Justin Hartley knew he was onto something special the moment he read the script for NBC’s breakout hit “This Is Us.”

He knew that the series, which has its season finale airing Tuesday, was groundbreaking and that it was going to change lives. What he didn’t know was if people would watch it.

Pilots come and go, and for every show the network picks up, there are countless that never get aired. And of the ones that do make it to broadcast, many won’t make it past the first season, some even dying after two episodes (see “Doubt,” which aired on CBS on Feb. 15 and 22 before getting canned).

“Pilot season comes and you read a bunch of pilots — if you’re lucky — and if you’re super lucky, you get a few auditions, and if lightning strikes, you get cast,” Hartley explained to amNewYork. “And if lightning strikes twice, the show gets picked up. And it never happens where the show gets picked up and so many people watch it and it’s critically acclaimed.”

THE ROLE OF A LIFETIME

Hartley says that Kevin Pearson is a character that he recognized, and it was a “role that I had to do.” In the first episode, Kevin, sharing a scene with the late Alan Thicke (“This is the most charming, energetic human being I’ve ever met in my entire life,” Hartley says. “I fell in love with him after like, three minutes.”), has a meltdown on set and quits “The Manny” in front of a live studio audience.

“My first impression of him (his own character ) was probably something a little different than most people’s first impression of him,” Hartley says. “I’m maybe a little bit closer to that, to understanding him, than some people would be. I understand at first glance it looks like this guy who’s sort of an ingrate and he’s got all this money; he’s got this great job and whatever. ... It’s like, ‘What are you complaining about?’

“But for me, I immediately was able to tap into how he felt empty and why do people fill their lives with things like that? There’s a gaping hole there, right? And when you get further into the story, you realize, and you start peeling back these layers of Kevin, you realize how deep this guy is. … He’s a well-intended guy. But yeah, I sort of latched on to that.”

Kevin is part of “The Big Three,” a moniker that his parents, especially his father Jack, would call his trio of children. (Kevin’s siblings are played by Chrissy Metz and Sterling K. Brown.) The three actors have wonderful on-screen chemistry, which Hartley attributes to a collective love for the script, something he says bonded them together.

“We’re all professional actors, but at the same time ... you’re like, ‘I can’t believe I get to work on this,’” he says. “Everyone was just so excited and so in love with the script … and that’s what brings everyone together.

“I read a script and I’ll be like, ‘Damn that was good,’” he continues. “And then I’ll hear Chrissy read the lines that I’ve already read in my head — or Milo [Ventimiglia], Sterling or Ron [Cephas Jones], whoever — and it’s at that moment where you’re like, ‘There’s no one who could play that role other than the people that are playing those roles.’ There’s just no one that could do it like that. They’re so good.”

A SENSE OF THE FAMILIAR

Hartley, like Kevin, has lots of siblings. His older brother is a police officer near Chicago and he has two younger sisters, one working in the business world and the other in the arts, he says.

Like his on-screen relationship, he says that he’s also got a strong bond with his real-life siblings.

“We’re all very close,” Hartley says. “One of the things about having close siblings is you can call and kind of pick up where you left off even if you haven’t talked in two weeks. … There’s a comfortable, like familiarity, an ease about a sibling relationship, I think. But it can also be very, very difficult. I mean I think you talk to your siblings sometimes in ways that you wouldn’t talk to a complete stranger and these are the people you’re supposed to love.”

A Knoxville, Illinois, native, Hartley first made his mark in acting in soaps, beginning with the NBC daytime series “Passions” as Nicholas Foxworth Crane from 2002-06. It was a job that would give him a strong work ethic over his more than 450 episode run.

“I think I did 160-something pages on my last day,” he recalls. “It was like eight episodes in two days. It was ridiculous. It was too much.”

His next big break would come playing the superhero Green Arrow on “Smallville” for more than 70 episodes from 2006-11. He had runs on “Emily Owens M.D.,” “Revenge” and “Mistresses,” and also took some more work in the soaps with 100 episodes of “The Young and the Restless.”

IN IT FOR THE LONG HAUL

Despite just a single 18-episode season of “This Is Us,” he sees this as a role he’d like to keep playing for the long haul.

“As long as it continues to be what it is now, I’ll play it for the rest of my life,” he says. “It is intriguing and it’s moving. It’s touching. There’s a story to be told. I’m in the happiest place I’ve ever been in professionally. So I’ll play him as long as they let me.”

And it’s very likely that based on the success of the first season, “This Is Us” could be around for a while. It has already been renewed for a second and third season, and Deadline.com reported that last week’s episode set a series record for total viewers. It’s pretty likely that Tuesday’s season finale could set another record. So far, the series also has been recognized with three Golden Globe nominations, one for best television drama, and both Chrissy Metz and Mandy Moore landing competing best supporting actress in a drama noms.

Nominations for the 2017 Emmy Awards won’t come until July 13, but there’s a good chance that the “This Is Us” stars will be up for some hardware.

Hartley took a more lighthearted approach to the awards seasons and the nomination process in general.

“I don’t know how it works,” he jokes, “But I know that I will flip a table if I don’t get nominated for something. You know what I should do? I’ll do a ‘Manny’ meltdown.”