Ventimiglia, who plays Jack Pearson on the series, was asked to share details about a "twist" in the Season 4 premiere that his co-star Mandy Moore previously teased.
"The world expands," the star said.
"It's actually harder to tease," he added of keeping the show's secrets.
"[The network has] never chastised me or slapped my hand or anything. [The cast] kind of dances next to that line but they never really cross it."
Fans have been speculating since July when Moore, who portrays Jack's wife, Rebecca Pearson, said Season 4 begins with an unexpected twist.
"We're starting Episode 1 and posted a picture [on social media] so people know we're back in the early days of Jack and Rebecca's courtship," the actress told The Wrap.
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"Like, right off their road trip to California, back in Pittsburgh. And we'll see how they are going to sort of navigate their lives at home and introduce one another to each other's families."
"But there is a twist at the end of the first episode that even I didn't see coming and I feel like it's, not to build it up too much, but it feels analogous to the pilot where it's like, 'Whoa, I was not expecting that!' It's so beautifully done," she said, referencing how the pilot shows how Jack and Rebecca's story is set in the past and that Kate (Chrissy Metz), Randall (Sterling K. Brown) and Kevin (Justin Hartley) are their adult children.
Ventimilgia said during a game on "WWHL" that he cries every time he watches the "This is Us" pilot.
"It's the moment when you realize that they're all a family. The camera moves and it's 1980 and it just swells," he said of his tears.
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