Destin Daniel Cretton wrote and directed “Short Term 12” based on his real-life experiences working with at-risk youth. (Urs Flueeler/EPA)

Ask Destin Daniel Cretton about his first day of work at a short-term home for at-risk teenagers, and the first thing he does is take a deep breath. Then he says: “Oh, man.”

He begins: “My first day, it was frightening. It’s like, all the natural nervousness of starting a new job, but with the added nervousness that comes with understanding that if you mess up, you can really damage another human being, you know?”

It was 2001 and he’d been looking for work for a while, after earning a degree in communications from Point Loma Nazarene University in San Diego. His friend was working the overnight shifts at this place, the group facility, and this friend told Cretton they were hiring. Cretton “went in and got the job pretty easily,” he says. A quick training session, a few days shadowing people and boom — he had a job tending to some of the most volatile, vulnerable and potentially dangerous people he’d ever met.

Cretton remembers, now, how he “initially walked into that job with . . . a kind of unhealthy savior attitude of — at the time, it was with the best intentions — but it was a very naive outlook on that world and the entire world in general. I thought I’d go in there and be the cool guy who just makes life better for everybody.”

On day one, Cretton talked to a “super-shy” kid. Things seemed to click. “I was feeling pretty good about myself. It seemed like we were connecting.”

That is, until they weren’t.

“I don’t even know exactly what the specific trigger was, but he ended up getting really upset.” Like something out of a comic book, Bruce-Banner-to-Hulk style.

The kid went inside his room and slammed the door. BAM! Then he came out of his room with a plastic chair in his hands. He threw it across the living room. WHOOSH! It bounced off a plexiglass window “three feet from my head,” Cretton says. POW!

“That was my first slap in the face as to how complicated this world is,” Cretton says. “And how many gray areas there are in this specific world, and also just the world in general.”

Cretton didn’t know then that what he was experiencing would become the film “Short Term 12,” which he wrote and directed and that opens in area theaters this weekend. “It actually took a bit of distance to even consider that,” he says. “It was such a strong experience that it took a while for me to organize.”

Three years later, one year after he left that job, he was poring over old journals and looking for inspiration for his thesis film. Cretton came upon a journal entry about an experience he had with one boy. You know the type: hard as rock on the outside, a tangled mess within. A volcano in human form. “Like a lot of the kids,” Cretton says, he “put on an air of, ‘I don’t really give a s---, nothing can touch me.’

“His dad was supposed to pick him up on his birthday to take him out for the day, and he was acting like he didn’t care. And then his dad didn’t show up. And he was acting like he didn’t care, for a while, until we heard his door slam. . . . I went over. He was keeping his door closed, I was pushing it open, and he finally let it fly in and took out his aggression on my face. He says some of the meanest things to me I think any human has ever said to me.”

Cretton developed that event, and the epiphany that followed — “All of that anger that he took out on me had absolutely nothing to do with me” — into a 22-minute short, “Short Term 12,” which won the Jury Prize in Short Filmmaking at Sundance in 2009. The film’s tagline had that kind of matter of fact, my-life-sucks-don’t-think-I-don’t-know-it attitude of the characters it documented: “A film about kids and the grown-ups who hit them.”

Cretton attended film school at San Diego State University and turned “Short Term 12” into a feature-length film. The full-length movie got rejected from Sundance but, at South by Southwest this past March, it won the audience and grand jury awards.

Cretton hopes that audiences can in 90 minutes glean some of the insight he developed over two years on the job. “The biggest thing I learned while I was working there is that there isn’t a huge difference, at least in my experience, between . . . the people in charge and the people that are supposed to be being cared for. And initially I felt like I had to somehow prove myself as being the leader and being the one who has everything together, and I found that the best leaders there are the ones who . . . do not feel that they are on a higher plane than the kids. They are able to be open and honest and equally as vulnerable with them when it is appropriate to do so.”

“Open and honest and vulnerable” is a good summation of Brie Larson’s performance. The 23-year-old actress (seen on Showtime’s “United States of Tara,” “21 Jump Street,” “The Spectacular Now”) gets her first starring role as Grace, a counselor who helps people solve their problems while shoving her own strife off to the side, refusing to acknowledge it, maybe thinking it’ll go away if she just acts like it isn’t there.

“The Grace character is extremely guarded and has a lot of [stuff] that she hasn’t dealt with,” Cretton says. “In every single scene in the movie, she has something else on her mind.”

Grace is frustrating, which isn’t to say that she’s not likable — “I think Brie naturally brings that lightness, and reliability kind of naturally is in her,” Cretton says — but rather that, for every thousand thoughts that must be brewing in her brain, one, maybe two, are musings she opts to share. Female characters don’t usually get to be difficult like this; as a general rule, they have to be the girlfriend, wife or mistress who caters to her man’s crazy (see also: Betty and Megan Draper, Skyler White). This time, it’s Grace who gets to talk in those elliptical, infuriating sentences, Grace who withholds the information that would put her sometimes inexplicable behavior in context, Grace with the dark past she refuses to discuss. Her boyfriend, Mason, has to be the picture of patience, resolve and forgiveness. He bends to keep her from breaking. Mason is played by John Gallagher Jr., who can be seen reporting on the very recent past on HBO’s “The Newsroom” and is no stranger to troubled-teen territory; he won a Tony Award for his portrayal of the sexually frustrated, suicidal Moritz in “Spring Awakening.”

Larson was won over by the dialogue in Cretton’s script. “When you’re dealing with characters that have a hard time expressing themselves, it’s really a tricky thing to try and write, and it was beautifully captured,” she said. She shadowed at a facility and read Reddit AMAs, short for “ask me anything,” with people who have Grace’s job in real life to prepare for her role. “I found that overall, and this comes from the kids and the line staff, the human ability to live and to continue wanting to live and the strength that we have, that we can really overcome a lot. The stories that you’d hear from these kids were so devastating. I found it inspiring that they were as functional as they were.”

“Short Term 12” has that kind of “Perks of Being a Wallflower” problem pile-on that can be hard to stomach, not because it strains credulity but because it strains compassion: It’s a little draining, to care so hard about so many people whose lives have been so thoroughly wrecked by other people’s carelessness.

But Cretton’s film isn’t some laughter-free zone, nor was his experience on the floor. “It [isn’t] just a feast of negativity,” he says. “The movie also shows the quality as authentic love and bliss and happiness and humor that is wrapped up in the kind of world that also has a lot of tragic things in it.”

So we get Marcus (played by Keith Stanfield), a 17-year-old boy soon to age out of the Short Term 12 facility, who goes from playfully riffing on how Mason “think we don’t know about him and Grace on the low trying to date” to bitterly rapping about how he’s lived “a life not knowing what a normal life’s like.” We get cutters, but also cupcakes. We get invested in all these people who wind up deeply invested in each other, whether they planned to be, or want to be, or not.

“To me, the movie is all about family, but not necessarily in the traditional sense,” Cretton says. “It’s about the human ability to create that even when there isn’t a traditional family available. Humans are able to create that wherever you are. And they’re able to create it in the midst of really difficult, horrendously difficult experiences. I don’t know, I just think there’s nothing more beautiful than that.”