In the spring of 1985, journalist David Blum flew to Los Angeles to interview an up-and-coming movie star for New York magazine. While there, he caught up with friends for dinner. It was a gluttonous meal, and someone joked that their gang should have a name: Sort of like “the Rat Pack,” Frank Sinatra’s legendary posse — but Blum’s crowd was more like “the Fat Pack.”

Everyone had a good laugh. And later that night, a light bulb went on in Blum’s head.

The actor he was interviewing, Emilio Estevez, also ran with a tight-knit cadre of show-biz dudes who liked to party. Sort of like the Rat Pack, but younger, greener, brattier. More like. . . the Brat Pack.

We’re in the thick of Brat Pack nostalgia these days, midway between last year’s 30th anniversary of “Sixteen Candles” and next year’s 30th anniversary of “About Last Night,” the movies that bookended the best years for the Reagan-era teen idols. This season has seen a wave of tributes for the most Brat Packiest movie of all, “St. Elmo’s Fire” — the 10th top-grossing movie of the classic summer of 1985 (“Back to the Future,” “Rambo,” “Fletch,” etc.).

But too often overlooked amid the memories of homemade prom dresses and saxophone riffs and sassy, jaw-grazing bob cuts is Blum’s stunning magazine article that branded this group of actors and the moody coming-of-age movies they were making at the time — the story that made the Brat Pack a thing.

Just go read it right now. It’s a funny, gossipy, uncommonly insightful work of celebrity journalism — the likes of which we haven’t seen in years. Among the fascinating scenes, we see:

  • Estevez with buddies Judd Nelson and Rob Lowe hitting on women at the Hard Rock Café – or rather, coolly assessing the throngs of ladies throwing themselves at them. (Estevez goes home with a Playboy Playmate.)
  • Estevez contriving to get free tickets to a screening of Matthew Broderick’s “Ladyhawke” (I know, I know: He needed free tickets? To “Ladyhawke”?) by calling the theater manager with a variation on the do-you-know-who-I-am routine.
  • Nelson dancing by himself at a nightclub until he realizes no one is watching him, let alone joining him. (Blum writes: “So after a few minutes, the anonymity appeared to be too much for him; he sat down with a dejected look and started complaining about what a horrible club it was. Then he suggested they leave.”)
  • The gang continuing its evening at an after-hours punk club, where they are immediately recognized and whisked to the front of the line, but quickly get bored and leave again, casually ditching their guest for the evening, “Bright Lights, Big City” novelist Jay McInerney.
  • Estevez icing out his Academy Award-winning peer Timothy Hutton during an awkward encounter at a Westwood pizza stand. (“Tim’s last three movies were bombs,” an unidentified Brat tells Blum. “It’s going to get to the point where the Oscar isn’t going to matter. If you can’t sell tickets, that’s it.”)

The story doesn’t cleave perfectly to your hindsight notions of the Brat Pack era, however:

  • It includes Tom Cruise, Sean Penn and Nicolas Cage as Brat Packers. (I know, I know. But it was completely reasonable at the time: None had Oscar nominations nor a “Top Gun”-size hit yet; they were still on the same rising-star tier as the “St. Elmo’s” guys.) Also, Matt Dillon.
  • It dismisses Andrew McCarthy as a “New York-based” lesser member who didn’t socialize with the Hard Rock crew. (“A co-star says, ‘He plays all his roles with too much of the same intensity. I don’t think he’ll make it.’” Meow!).
  • Missing from the story? The women we think of as Brat Packers. The focus is entirely on the guys — particularly Estevez, Lowe and Nelson, the off-screen amigos who ended up on New York’s cover — with no mention of Molly Ringwald and Ally Sheedy who have nonetheless spent the past three decades trying to peel the “Brat Pack” label.

But it remains, 30 years later, a startlingly fresh and captivating read — especially compared to today’s celebrity profiles.

These days, your typical celebrity interview takes place in the neutral territory of a restaurant or a hotel suite. It probably lasts less than an hour. A publicist is often sitting on hand the entire time. If the writer is very lucky, he or she might get to interview the star during a walk through a park arranged for the sake of giving the writer a bit more action to observe and describe, or maybe just in a car on the way to somewhere more important. Or sometimes they just talk on the phone. We only get to hear what the star says, and even then in a somewhat controlled environment; we don’t get to see how they live.

But Blum really hung out with Estevez and Co., in casual, real-life settings you can’t imagine a journalist being allowed into today.

How did he get that kind of access? Clearly, Blum was a very skillful reporter. But he was also working in an era when celebrities were much less cautious than they are today.

Blum, now the editor of Amazon’s Kindle Singles, said that he didn’t set out to write a story about the Brat Pack. His mission was to profile Estevez, then considered one of his generation’s most promising multi-talents. (I know, I know. But at 23, Martin Sheen’s son already had a deal to write, direct and star in his own film, like a young Orson Welles, and everyone was very impressed.)

“I was interested at that time in exploring the nature of fame,” said Blum, “and also the nature of being an artist in the modern age.”

So Blum met Estevez for lunch one day at the Hard Rock Café, he recalled, “and we hit it off to the extent that he said, ‘Why don’t you come here and meet me for dinner with my pals?’”

(I know, I know. The Hard Rock Café? But it used to be cool! In 1985, there were only three in the U.S.; the one in L.A. had been the first to open, and it was still a hot new place.)

The pals turned out to be Lowe and Nelson — Estevez’s “St. Elmo’s Fire” co-stars. In his 2011 memoir, Lowe explained that Estevez asked them to come out that night: He was concerned that the journalist had “only seen his serious, hardworking side and [was] eager to show him that he can have fun as well.” Some “fun girls” were invited, Lowe wrote, “in case the writer is single.” Toasts were made and drinks thrown back, as you might expect for a bunch of twentysomething single guys; filming had wrapped not long before, and they were “still in some way living out our characters’ relationships.”

For Blum, “that night crystallized everything,” he said. “The behavior I first noticed that night I kept encountering through the week.” It provided the opening scene of his story, and it inspired him to broaden the focus from a simple profile of Estevez to a larger anthropology of this new breed of stars — actors who were finding fame young and fast without years of theatrical dues-paying, many of them having grown up in showbiz.

Blum’s bosses at New York loved the idea. The only problem was that this was not the idea he had pitched to Estevez’s publicity team. But the magazine’s editor at the time, Edward Kosner, “was very bold and didn’t care about the Hollywood publicity machine.” When the actors learned, to their horror, that Blum was writing about all of them, instead of just Estevez, they declined to participate in a photo shoot. No problem: Kosner just grabbed one of the publicity shots from “St. Elmo’s Fire” and used that for the cover.

When the issue hit the streets in early June, it made the kind of immediate zeitgeist splash that was rare in the pre-Internet era. “When I saw it, I just knew, that’s it,” Nelson told author Susannah Gora, for her 2010 Brat Pack history, “You Couldn’t Ignore Me If You Tried.” The actors, busy making the promotional rounds for the June 28 release of “St. Elmo’s Fire,” were asked about it by Phil Donahue and David Letterman. It was Topic A in a Washington Post profile of Estevez that month. By the end of 1985, “Brat Pack” was part of common parlance.

But the Brat Pack hated the phrase. In fact, they hated everything about the story — “a sneak attack, mean-spirited hatchet job,” Lowe called it — and went on the offensive. Some claimed that Blum had violated the terms of an off-the-record dinner — which Blum strongly denies.

“Yeah, I went into it to write about Emilio Estevez, but aren’t journalists supposed to write what they observe? And I observed these other things,” he said. “It’s called a profile, which means technically that you’re writing about someone from a specific angle — literally, getting a profile.”

Clearly, the story put a new chill in the air for a certain set of movie stars. “What I learned is the press is never really your friend,” Estevez groused to The Post in that June 1985 story. “Now I’m a lot more guarded.”

So was Blum’s story the cruel turning point for celebrity journalism? Is this what prompted Hollywood publicists to lower the boom on reporters and limit access to tightly controlled meet-and-greets?

Blum acknowledges that we don’t see stories like this much anymore. But he doesn’t think there was a direct cause and effect. “Even I was able to continue to write about Hollywood for some time after that, without the stars running away from me into the street,” he said.

Instead, the rules slowly changed. Vanity Fair led the way, Blum said, by assembling dazzling cover packages — sophisticated writing, Annie Leibovitz photos — that often required little more than a lunch interview from its subjects.

“It became a formula, and it was done well,” he said. Vanity Fair “redrew the lines” of engagement, and then it became harder for journalists from other magazines to get any more access to the stars. Which has led to all those stories that analyze the way Julia Roberts picks at her shrimp Caesar salad, or whatever.

For Blum, the final straw came about a decade after the Brat Pack story. Weary of the strictures of showbiz publicists, he set out to do an in-depth profile of Johnny Depp for Esquire — interviewing the star’s friends and colleagues, even consulting the police report from his famous hotel-room-trashing incident — without actually talking to Depp. But then the actor’s publicist called: If she could get Esquire an interview with Depp, would they do it as a cover story? He couldn’t say no. Still, he was pleased with the story. But at the photo shoot, when Depp’s publicist left the room, the photographer nudged the star to take his shirt off — snap! It became the cover, of course. And suddenly the publicists were in an uproar again, and all anyone could talk about was shirtless Johnny Depp — not the story.

“I don’t think I ever wrote about Hollywood ever again,” Blum says. “I just felt like ‘I can’t do this anymore, I can’t play this silly game with publicists and celebrities and photo sessions.’ … I didn’t think I had anything more to learn or convey about it as a writer that I hadn’t explored.”

And though the genre has faded, Blum said it’s not extinct. He’s a fan of Lynn Hirschberg’s celebrity profiles — particularly her 2013 New York magazine story on Michael Douglas, for which she seemed to get under the actor’s skin as well as into his home.

And he has great praise for a Vanity Fair story — Buzz Bissinger’s profile of Caitlyn Jenner. “That was deeply reported with phenomenal access,” he said. “They could have done it in an easy one-interview way, but instead they gave it to a prize-winning journalist. … That’s a gigantic win for everyone, so maybe it will lead to more access.”

Which is something the stars should consider, he said. “The more you restrict access, the more you make journalists suspicious of you. If you really trust yourself, you might as well give them an opportunity to really understand you.”