When Elliott, played by Maisy Stella, skips a family birthday dinner to take magic mushrooms with her friends in the woods, it may feel like a set-up for a coming-of-age comedy. And on one level, “My Old Ass” is precisely that: a funny look at Elliott at a turning point in her life when she meets Chad, played by Percy Hynes White, just before she leaves her small-town home for college in the big city.

Yet at the Sundance Film Festival this year, audiences were weeping, too. The movie’s hook is that, as she’s tripping, Elliott meets an older version of herself played by Aubrey Plaza, who offers some hard-to-follow advice to her younger self. As old and young Elliott — who manage to maintain a connection even after the drugs wear off – face up to what life has in store, the laughter turns to poignant tears.

The film, in theaters Sept. 13, is just the second by Megan Park, an actor whose acclaimed writing and directing debut, “The Fallout” concerned a high school student (Jenna Ortega) struggling with trauma after surviving a school shooting.

Park spoke by video recently about capturing the language of teens and finding the story in the editing room. This interview has been edited for length and clarity.

Q. Did you start with the idea of a teenager at a pivotal moment or the conceit of the mushroom trip and meeting your older self? 

As a writer, I’m emotion-led. “The Fallout” came from my frustration and anger at the idea of American high school students having to constantly exist with the possibility of school shootings. For this movie, I was home in my childhood bedroom during the pandemic and had had a baby and I was feeling nostalgic. I had that feeling that comes in the scene where Chad talks about the last time you play with your friends as a kid and you don’t realize it’s the last time. That made me want to explore that idea of the older and younger self. The mushroom trip idea came when I was trying to figure out how to make that happen.

Q. Among the movie’s many fantastic qualities is dialogue that’s pitch-perfect for teens. Do you have a natural ear or did you give your young actors input? 

I started out acting, so I’m a stickler for dialogue and when I’m writing I say it out loud to make sure it flows. You have to know what you know and what you don’t know.

I try to ground it and make it authentic, but I’m not 18 so I try to be open-minded and try to really include the actors: “Is that joke funny or is there another phrase you’d say instead?” It’s an open line of communication every step of the way and they’re so helpful and always checking for me how relatable it is.

And we did scripted takes but also a lot of “fun-runs” and the chemistry between Maisy and Maddie Ziegler and Kerrice Brooks as her best friends was really natural and created great moments organically.

Q. Do those improvised “fun-run” moments end up on screen or is it more that they create chemistry that fuels the scripted material? 

It’s both. A lot of times it’s the runway to get the script down but there were moments that stayed in, especially ones that just started with Aubrey saying something and then they’d go off. And it was Kerricet’s first movie but she’s so good that I just kept saying, Put her on her mark and just hit record and see what she says.

Q. What were you looking for in casting for Elliott and Chad? 

We were really lucky because the finance people and producers said find younger Elliott first and didn’t insist on just finding the hottest property. I wanted someone very grounded and very Gen Z, but who had a vulnerability and also a lightness and joie de vivre. Then we cast around her for chemistry with everyone. Percy sent in a self-tape and it was genius and he just understood the humor. When he did the slate with his name, everyone had to show their full body, so he was wearing a nice shirt and then panned down to show nice pants … and bare feet. It was so Chad.

Q. There are plenty of laughs and a very silly hallucination involving Elliott performing as Justin Bieber. How hard was it to find the balance between humor and pathos knowing where the movie is headed?

It was the hardest thing. We had hoped the movie would be heartfelt but the script was lighter and it was that the performances were just so incredible and the location had such a beautiful and nostalgic feel that in the edit the movie became much more heartfelt and emotional than we even expected. So there’s a delicate line and you have to go back through each moment and reverse engineer and think of how much of a door do you open with each scene. Once we really discovered what the movie was, we were able to change things in editing with the advice that Aubrey gives to Maisy in their phone calls.

Q. Did you know the reaction you’d eventually get once the movie started screening?

No. With humor, you can get a gauge as you’re doing it, but with emotion you can’t always tell because you’re in so deep while you’re filming and you’re worrying about things like whether the camera is in focus. Although I did get pretty emotional when we were filming — Aubrey’s performance really killed me.

Still, you don’t know, and in the edit, you watch it 7,000 times so it’s hard to say for sure. Sitting in that audience at Sundance with all genders and ages having such a universal reaction was pretty insane.

When John Carpenter’s “The Thing” was released in 1982, it bombed.

Steven Spielberg’s “E.T.,” by contrast soared, becoming a national phenomenon. Still, the two share a bond in Chris Nashawaty’s new book, “The Future Was Now: Madmen, Mavericks, and the Epic Sci-Fi Summer of 1982.”

The other films in the book are “Star Trek: Wrath of Khan,” “Tron,” “Blade Runner” and several others loosely connected to the genre: “Poltergeist,” “Mad Max: The Road Warrior” and “Conan the Barbarian.” Nashawaty tells how each film’s director got there and how each film was made, piling on a full platter of memorable anecdotes: “Poltergeist” was developed from an early version of “E.T.” Leonard Nimoy, after the disappointing first “Star Trek” film, only came back for the sequel because he was promised a good death scene. Arnold Schwarzenegger nearly got torn to shreds by dogs – who had been bred from wolves – while making “Conan.”

Nashawaty spoke recently by video about the films, the genre and his favorite stories. This interview has been edited for length and clarity. 

Q. How did you come up with the idea?

I was 13 in the summer of 1982 and saw all these movies in the theater and I have fond memories of that summer. Then I was writing an essay for Esquire two years ago about the anniversary of the date both “Blade Runner” and “The Thing” were released. They’re regarded as classics now but were beat up on by critics and neither one did that well. People reacted strongly to it so I looked back at that summer and saw it was weekend after weekend, one big sci-fi movie after another. 

Q. How do you define sci-fi and how do you think sci-fi fans and the general public will? Because I don’t think all of these are sci-fi movies. 

You’re right, it’s a very subjective definition. The genre means different things to different people. Maybe it was the video store I went to when I was a kid where sci-fi was right next to fantasy so they get grouped together, which was my loophole for including “Conan the Barbarian.” I just wanted to write about John Milius and Arnold Schwarzenegger. “Poltergeist” is a horror movie, but began its development as a science fiction movie. “Road Warrior” is an action movie, but I think it fits into the genre. I’m a little bit more elastic with it than other people. I feel like anyone who’s passionate about something that is speculative about the future or even an alternate present will be open to this. It feels right to me. 

Q. Which were your favorites in 1982 and what about now? 

In 1982, my favorite was clearly “E.T.” To me, that was like looking at the Sistine Chapel for the first time. It was a pretty massive experience. I would say now my two favorite movies of the bunch are “Road Warrior” and especially “The Thing.” There’s something about that group of guys in this sub-zero outpost, and the way John Carpenter builds the tension and the amazing special effects. It’s just one of those movies that really speaks to me. 

I wasn’t a huge fan of “Tron” before writing the book but I now have a real respect for it. It’s really a prototype for basically CGI and animation. I don’t think it’s the greatest movie. And the effects look a little dated now, but if you think about how they made it and what they did with it in 1982, it’s pretty staggering. 

And this is controversial, but I realized that I really like the voiceover version of “Blade Runner” better than the director’s cut, because it’s the one I saw first. I like the film noir aspects of it. It feels like “The Maltese Falcon.” I agree that it’s a better movie without the voiceover, but only after you’ve seen it with the voiceover, so you understand what is happening. 

“Conan” has always been a guilty pleasure for me. You’ve got to approach it in the right way to really enjoy it. The music is old-fashioned and it’s cheesy but it’s a real epic. There’s something about the swaggering macho old-school comic book feel I really like. It has a certain appeal for the inner 12-year-old boy in all of us. You know what I mean? 

Q. My inner 12-year-old boy would rather watch “The Bad News Bears.” 

Fair. So would I.

Q. What are some of your favorite stories from the book?

The one where George Miller wanted a three-legged dog for “Mad Max” and they couldn’t find one so he suggested they amputate a healthy dog. It just makes you go, “What the hell?”

I loved Nicholas Meyer, who wrote basically the entire “Wrath of Khan” script in five days without getting paid for it, just because he wanted to direct it. He really impressed me. 

I’ll be honest with you, the book I wrote before this was about “Caddyshack,” which had lots of cocaine stories. So I really liked talking to Oliver Stone about his foggy period while he was writing “Conan.” 

And I love Philip K. Dick finally going to see a screening of “Blade Runner” and being surprised that he loved it. Which was right before he died. 

It’s also really fun seeing what projects didn’t happen, like [Ridley] Scott making “Dune.” Or Dustin Hoffman almost starring in “Blade Runner,” things that you would never expect. 

Q. In the end, you say these eight movies rewrote the rule book and that changed the paradigm for Hollywood. But I think the better argument is that from 1975-1982, Spielberg, George Lucas, Scott and Carpenter did that with movies spanning adventure, horror and sci-fi. Can you make the case for 1982? 

You could go wider. That’s sort of what I tried to do by giving the background of “Jaws” and “Star Wars” and what each of these filmmakers were doing building up to 1982. But I feel like this was the moment Hollywood realized there was this audience of fans who would go back and watch the movie a second, third and fourth time. So they said, “OK, we’re going to cater to you” although it eventually became movies made without the same creativity and just for the money. These movies were a moment when a completely new audience was deemed important to cater to. That’s the change. I think it started good and then it went bad.