'This Is Us' cast has known about that huge premiere reveal for a long time

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(CNN)Warning: This post contains spoilers about the season premiere of 'This Is Us.'

You wanted the truth about Jack's death, "This Is Us" fans. But can you handle the truth?
In the first episode of the sophomore season, "This Is Us" unveiled a huge piece of the "How did Jack die?" puzzle in a flashback to the night of his death. In the scene, a distraught Rebecca (Mandy Moore) sobbed in her car parked in front of the Pearson house, which was burned down to its frame, indicating Jack's death is somehow related to a house fire.
Before you ask, no, this isn't a misdirect.
    "That is the cause of his death," "This Is Us" creator Dan Fogelman told CNN at a party following the show's Los Angeles premiere screening. "[The fire] is directly responsible."
    Still, many pieces of this puzzle remain unsolved.
    For example, leading up to the reveal of the burned house, viewers saw Rebecca, wearing a Steelers shirt, driving with a bag of Jack's belongings in her passenger seat. Does this mean Jack made it to the hospital? And what was the status of Jack and Rebecca's marriage at the time of the fire?
    In the season premiere, Jack (Milo Ventimiglia) admitted to his wife that he had a drinking problem, and Rebecca vowed to see him through it.
    The flashback also hinted at major developments in the lives of teenage Kate, Kevin and Randall. Kate, for example, had a dog, Randall had a girlfriend and Kevin had broken his leg.
    Fogelman promised all these small details will add up to a final picture that will fully explain the events leading up to Jack's end, but it will be "a slow-burning mystery."
    "Hopefully, we've given people enough that they can focus on the show -- understand the big piece and, now, be watching a little bit of an onion unpeel as you see things ensue in the course of the [season]," he said.

    Secret's out

    The final minutes of the premiere were the result meticulous planning. In fact, what fans saw in the Season 2 premiere was "very much the plan since [the show's] inception," Fogelman said.
    "Milo, Mandy, Sterling [K. Brown], Justin [Hartley], Chrissy [Metz] -- they've all known since day one of shooting that this is what happened. They kind of knew this is how the first episode of the second season would start," he said.
    Keeping the deadly fire a secret this long has not been easy.
    When filming the all-important burned house scene about a week and a half ago, the "This Is Us" team traveled several hours out of Los Angeles in hopes of keeping it under wraps. They built a replica of the Pearson house, used code words and kept anything relating to the show hidden from public view as much as possible.
    Fogelman estimates it was six months of planning total. Though, he admitted in a panel after the screening, filming took mere minutes, as Moore emotionally delivered what they needed in the first take.
    "She's so good," he said.
    With the secret out in the open, Fogelman said he's breathing a little easier.
    "The big thing we were protecting was [Tuesday night's episode], and now it's out there and I'm less worried," he told CNN.
    Mandy Moore as Rebecca on "This Is Us."
    The fire theory had been one of many floated by "This Is Us" fans since the mystery of Jack's death was built up in Season 1. Easily the most popular theory had Jack dying in an automobile-related incident, a hypothesis born from one scene in particular last season, involving young Kate and drunk driving Jack.
    "We had a moment in our second to last episode where Kate reveals — which is very much at the core of our story — that she feels responsible for her dad's death as he gets into a car drunk to drive, and there was some blowback that we misdirected the audience," he said. "In our minds, it was something that happened. Jack did drive having drank that night, and she does feel responsible for her father's death, which is something we're going to see this season. But I understand that could have felt a little bit manipulative. That wasn't our intention."
    So this season, they're being extra "careful," he said.
    "Everything that has been said on the show regarding Jack, everything that they've seen [Tuesday night], is not a fake-out," he promised. "People will see it."
    Also in store? Some major singing moments from grown-up Kate (Metz), more from Randall (Brown) and Beth's (Susan Kelechi Watson) adoption journey, a guest appearance from Sylvester Stallone and a deeper dive into what made Jack a loving, caring father and husband.
    "There's always a darker sided to the lighter side," Ventimiglia teased. "There are reasons why he does these things and that's what we're going to unpack."
    "This Is Us" airs Tuesdays on NBC.