Milo Ventimiglia tells PEOPLE why he "loved" filming the "After the Fire" episode, especially putting on future Jack Pearson's "slight little belly"

WARNING: This post contains spoilers from Tuesday’s episode of This Is Us.

This Is Us just answered the question: If Jack Pearson (Milo Ventimiglia) had lived, how different would life be?

Tuesday’s episode, titled “After the Fire,” presented scenarios, through Randall’s (Sterling K. Brown) perspective, of the patriarch surviving the house fire that caused a fatal widowmaker heart attack due to smoke inhalation.

Two versions starred two different Jacks after wife Rebecca (Mandy Moore) confessed to knowing the identity of their son’s birth father. The rose-colored account saw teen Randall (Niles Fitch) immediately find William (Ron Cephas Jones) and get him drug-free. Jack and Rebecca’s marriage survived her lie and William became part of their future. Randall still attended Carnegie Mellon University, where he met Beth and produced the same stable relationship. In his adult years, Randall detected William’s stomach cancer early and worked with Jack to put Rebecca into the Alzheimer’s clinical trial.

In stark contrast, the version that scared Randall the most saw William once again rejecting his biological son; Rebecca’s lie affecting her trust with Jack and increasing his drinking; his parents’ marriage being rocky at times; a mother-son relationship growing estranged. Thus, Randall attended Howard University early, becoming a professor at his alma mater and philandering with his female TAs. Oh, and Kate (Chrissy Metz) was a mother to twin girls with husband Ethan — who’s Ethan? — and Kevin (Justin Hartley), who wed Sophie, took over the Big Three family construction business instead of becoming an actor.

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Credit: Ron Batzdorff/NBC

All the while, the one trying to understand Randall’s imaginative outcomes was his therapist Dr. Leigh (Pamela Adlon), who tried to advise him to acknowledge that “even if [Jack] had lived, your life could’ve gone a million ways.” To put it simply: Randall can’t control the outcomes of his life.

That realization made him want to put Rebecca in the nine-month St. Louis clinical trial even more, knowing that he couldn’t save Jack and tried with William. “I lost my father and I couldn’t stop it. I lost a second father and I couldn’t fix that either. Now my mother is sick and there are options,” Randall told Dr. Leigh. “I already lost three parents, losing my mother would break me. I will do anything to keep that from happening, I’ll do whatever it takes.”

In the end, he was able to convince Rebecca — over the phone by reminding her how good of a son he has been — to sign up for the trial, without consulting Kevin and Kate.

Before fans start dividing into team Kevin and team Randall after finding out the possible cause of their upcoming rift in the season 4 finale, find out why Ventimiglia “loved” filming the penultimate episode as he tells PEOPLE about putting on future Jack’s “slight little belly” and being in storylines that even viewers never imagined for his character.

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“On episodes like this, I was excited to work with everybody else,” Ventimiglia says, laughing. “I’m the most isolated on the show, usually it’s just Mandy and I. So when all of a sudden, I’m in a scene with Justin, Sterling and Chrissy, even Susan [Kelechi Watson] and Sulli [Chris Sullivan], it’s kind of exciting to me. I never get that chance.”

Adds the actor, 42: “I knew it was coming down the pipe. It wasn’t anything I was asking for but it was definitely something I was looking forward to when I knew the scripts were coming out.”

Along with being in scenes with most of his costars, Ventimiglia also enjoyed being in the makeup chair next to his onscreen wife as they underwent aging transformations.

“Actually, the best part of it was looking over and seeing Mandy. And I’m like, ‘Oh, Mandy’s going through the same thing and I’m right here with her and she does this all the time.’ I applaud her for that,” he says. “The process and the artistry behind it is really incredible. The effects team and Zoey Hay, our head of department, had two guys that were working on me and one’s an Academy award-winning makeup artist and the other one’s an Emmy-winning makeup artist. So it’s fun to sit in a chair and really watch and feel the way their process when makeup is happening, the process of their job is happening. I don’t mind it, I don’t mind it at all.”

The Emmy-nominated star wore prosthetics to show him all the way until age 74. “I like taking on another persona and another side of Jack, really embracing it. From the makeup and the clothing and the slight little belly that he has, I love it,” Ventimiglia says.

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Through Randall envisioning what life may have looked like if Jack were alive, fans were able to see a world in which Jack became a grandfather for the first time, toasting to his sons at their wedding rehearsal dinners and celebrating Thanksgiving in Randall’s Philadelphia home for the first time ever.

“It’s a question that everybody really may think about often,” Ventimiglia says. “It crossed over into our own lives. There’s always desire again to have the things that we can’t have, or to want the things that we can’t have. So knowing that we only have limited time with Jack in Jack’s lifetime, getting a glimpse of what could have been, I think it’s going to be pretty satisfying to a lot of people.”

“We played both sides of the coin all through Randall’s point of view,” he adds. “So I think hopefully people will take the more positive path with Jack and hope that they would have seen him in a better capacity than maybe the latter, the second situation that we saw him in.”

Of the second half of the episode, Ventimiglia explains it was a “different approach from Randall,” one in which he was “being really hard on his mom and really distant from his mom, just not quite as forgiving.”

“It takes her illness for him to forgive. I think a lot of people are like that in life,” he says. “They’ll go hold on to something until really it’s like you realize that life is really short and it’s best not to hang on to anger or hatred or anything like that.”

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Credit: Ron Batzdorff/NBC

All in all, Ventimiglia hopes fans learn to take a “positive approach” in their lives.

“People wonder what would’ve happened to have other decisions been made. This was one of those episodes, a good reminder for people to think about the decisions that we’re making and the commitments we’re making to those decisions, to our future and what’s really going to be the end results of it,” he says, later adding, “The great part about This Is Us is it sparks a lot of conversation.”

“So I go back to that practical application of learning from the show and know that maybe we’re influencing people to positively move past their differences with someone, or to just be more positive in their decision-making process,” he says. “So hopefully, that’s what I really want the takeaway to be is that a person is going to take the positive approach in their lives and things that are relative to their lives that inspire by the show.”

As for what to expect in the season finale, Ventimiglia couldn’t choose just one word to describe it.

“I don’t know if there’s a word so much as just get ready. Just get ready,” he teases. “It’s heavy, it’s beautiful and it’s really incredible.”

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