'This Is Us' Star Milo Ventimiglia Reveals Whether Old Jack Comes Back in Season 3 (Exclusive)

The 40-year-old actor and co-star Mandy Moore tease Tuesday's anticipated season finale.

When one Jack mystery is solved, another one surfaces on This Is Us.

Fans have speculated for the past several days over the reason why Jack, who died from a heart attack after suffering severe smoke inhalation, is seen as an old man in a 30-second trailer for Tuesday's anticipated finale, centered on Kate and Toby's wedding. Though it's been teased that there is a "beautiful" reason for old Jack's presence in the episode, we needed more answers.

"You will know why I am in aged Jack makeup," Milo Ventimiglia told ET's Leanne Aguilera on Monday at the South by Southwest Film Festival in Austin, Texas. "I don't want to tease anything other than you've seen the photos. Just be excited and enjoy."

But could old Jack somehow reappear on This Is Us past the sophomore finale? Ventimiglia appeared to debunk that idea.

"I feel like that's maybe a one-time thing. It's only once in your life, like a particular blooming flower," he said of the possibility viewers may see him in old-age makeup on a more frequent basis moving forward. 

Ventimiglia also spoke about experiencing firsthand the process to get aged up: three hours of makeup and one hour to take it all off. "It's a lot, but you can't help but feel like you're in your 70ish years when you're in it," he said. "We were in between scenes and I was talking to someone and how [I was] standing, I couldn't get out of 70-year-old Jack."

Mandy Moore admitted she was "pleasantly surprised" to read in the finale script that her TV husband would be taking on a new look, so to speak. "More so because somebody else had to go through the torture of going through old-age makeup. It wasn't just me anymore!" Moore told ET at SXSW. "I think it's a beautiful way to punctuate the end of this season."

When asked whether it was surreal to see Ventimiglia a few decades older as Jack, Moore confessed she "loved it." "It sort of makes you wistful for what could have been," she explained.

While it appears old Jack will be a rare finale moment on This Is Us, Ventimiglia said it's "exciting" to think about the prospect of future versions of the show's core characters.

"[Creator Dan] Fogelman's intention was always to jump into the future and that was something we all knew going in. When we saw them jump into adult Tess and older Randall, that was where he always wanted to go," he said. "I don't know if the show always lives there, but I think understanding that we do exist in the past, it makes sense that we'd exist in the future as well."

The This Is Us season two finale airs Tuesday at 9 p.m. ET/PT on NBC.