LOS ANGELES — Mandy Moore and Milo Ventimiglia, stars of the NBC drama “This Is Us,” were on the set one day last month, sitting in their fictional family’s living room, as they discussed a fairly remarkable feat: how network TV can still create hits.

Ms. Moore said she realized “This Is Us” was gaining real steam after a couple of unusual things happened: There were raves from people who usually watch “the cool show on Amazon,” not over-the-air TV; then there was praise from, of all people, Sarah Silverman, the caustic comedian, who has taken to Twitter armed with adoration. (“I’m a puddle of tears,” she tweeted recently, among some profanities.)

“I’m like ‘Whaaat?’ I would never have thought she would have been watching,” Ms. Moore said.

Mr. Ventimiglia jumped in to add: “You’re, like, HBO. What are you doing on NBC?”

“This Is Us,” which airs on Tuesday nights, is the biggest hit for the broadcast networks since “Empire” sent shock waves through the industry two years ago. NBC has taken the bold step of already renewing the show for a second and third season, even though just 13 episodes have aired. And as with the meteoric rise of “Empire,” “This Is Us” has seen its audience growing week to week; the most recent episode drew 14.6 million viewers, according to Nielsen data covering three days of delayed viewing.

But can this show keep it up? Indeed, can any new show keep it up these days?

A big-hearted — and occasionally quite funny — drama, “This Is Us’’ centers on the Pearson family as it navigates any number of challenges, like death, dieting and sibling rivalries. The show jumps from decade to decade stretching from the early 1980s — when Jack (Mr. Ventimiglia) and Rebecca Pearson (Ms. Moore) are raising three babies — to a present that features three still-figuring-it-all-out 36-year-olds.

There are challenges ahead. American audiences have radically changed the way they watch TV, and laws that used to govern the industry have been rewritten. There is one recent phenomenon that “This Is Us’’ hopes to avoid: Shows that start hot have fizzled after a break between seasons. The 2015 finale of AMC’s “Fear the Walking Dead” had 10 million viewers, yet its second-season finale barely registered half that, according to Nielsen.

Likewise, USA’s “Mr. Robot” shed nearly half its audience between its first and second seasons. Broadcast network shows like ABC’s “How to Get Away With Murder” and “Quantico,” Fox’s “Last Man on Earth” and NBC’s “Blindspot” similarly saw big drops after fine rookie seasons. Even “Empire,” which still has strong ratings, has lost a quarter of its viewership this season.

Television executives offer a few theories: Several of those shows suffered creatively after strong starts; some shows deployed one twist too many, potentially exhausting viewers. And at a moment when there is more TV than ever, it’s harder to get an audience to return to a show it has already sampled. This is one reason many networks are beginning to turn to limited and anthology series.

This is a trend that is not lost on the people behind “This Is Us.’’

“I’m aware of it because it happens to me,” said Dan Fogelman, the show’s creator. “There’s a point in a TV show where I check out a little bit despite my best efforts. You know that feeling: You’ve got that big DVR list at home, and you’re suddenly not going back to a program that used to be first up.”

But Mr. Fogelman and executives behind his show are convinced that “This Is Us” has passed a series of stress tests that indicate it can avoid a serious drop-off.

“It’s already bucking the trend,” said Dana Walden, co-chief executive of the Fox Television Group, which controls the studio that produced the show. “It has weathered an incredibly difficult fall in the TV business with the election, presidential debates, a wildly entertaining World Series and a holiday break.”

NBC executives even argue that the holiday breaks have benefited the show as word-of-mouth spread and new viewers began jumping on board by binge viewing. Within NBC’s offices in Universal City, Calif., they have been referring to its performance in early December as the “Thanksgiving bump”: The two episodes that were broadcast after the holiday had a jump of more than a million viewers, according to Nielsen.

NBC’s decision last month to give “This Is Us” a two-season renewal was not just bravado. It was also a strategic decision meant to entice would-be viewers and assure them that the show is not going anywhere.

Jennifer Salke, NBC Entertainment’s president, said that as TV ratings had eroded, networks had been too trigger-happy in recent years, canceling shows that failed to hit traditional ratings benchmarks — even if the show had a loyal fan base.

“We’re all over social media to figure out who’s watching, and there’s a lot of negative sentiment that I heard that goes, ‘I really want to watch it, but they’ll just cancel it and burn us like they always do,’” she said.

The long-term investment for “This Is Us,’’ she said, was intended as an “insurance policy” for viewers who were reluctant to jump aboard.

NBC had its earliest indications that the series would be a hit last May when, out of nowhere, its trailer became a viral sensation months before the premiere. (Mr. Ventimiglia’s prominently featured bare butt may have had something to do with it.)

It was that same month, Ms. Salke said, that she thought it might be a hit for another reason: When she looked at lineups at the annual upfront presentations, she noticed programming slates overflowing with procedurals and crime shows.

As Mr. Fogelman put it: “Not everybody is a cop or a lawyer or a schoolteacher turned drug dealer. It’s just a story about people and more specifically this family, and I think people are seeing themselves a little bit in it.”

Mr. Fogelman said he had the 1983 dramedy and tear-jerker “Terms of Endearment” in mind when he was conceiving “This Is Us.” And the show is certainly emotional. (Critics have taken to calling it manipulative.)

“This is Us” has been especially popular among women, ranking as the third-highest-rated show for female viewers ages 18 to 34, behind only “Empire” and “Grey’s Anatomy.”

When asked why the show is popular right now, executives offered two explanations. One was tonal: At a time when TV has gone gritty, sophisticated and dark, audiences may have room for something else.

“I’m all for really dark art and dark TV and film, but there’s a point where people are craving a different kind of emotion at 8 or 9 or 10 at night, and maybe we’ve tapped into that,” Mr. Fogelman said. “They’re looking to hold hands with a TV show, and something about the show has done that.”

The other part? That probably has something to do with the volatile and overheated political climate right now.

“The uncertainty is in the air, and nobody knows what to expect in the next couple months, coming weeks and year ahead,” Ms. Moore said.

She said that, as a result, people were looking for “cathartic entertainment.”

Ms. Moore and Mr. Ventimiglia — both dressed as the Pearsons, circa the mid-1990s — said they didn’t foresee any loss of momentum for the show, despite industry trends. Is there anything that could happen plot-wise to set it off track?

“Murder,” Mr. Ventimiglia said, laughing. “I think there would have to be murder.”