Ever wonder what happens in the theater when no one is watching?

Each spring, artist Peter Waldner quietly donates his time and talent painting scenery for Shelter Island School’s musical productions, ensuring the young thespians look good when they step out on the stage.

He’s been doing it for years.

“I usually go and paint when no one is around. But the first time I saw the auditions, that gave me an idea,” Mr. Waldner said.

His idea was to make a film about the school’s annual spring musical, documenting the process from student auditions with director John Kaasik, to final bows on opening night — and everything in between.

The result is “Behind Curtains,” an hour-long film about the school’s 2017 production of “Curtains,” a musical parody of a 1950s play-within-a-play whodunit theater mystery. On Friday, November 24, “Behind Curtains” will be screened in the school auditorium, providing an opportunity for cast members who graduated last June to see the film while they are home from college for Thanksgiving weekend.

The production was well covered. Mr. Waldner shot 120 hours of raw footage which he turned over to his longtime friend Bob Volpe, who edited it down to a manageable length.

“We’ve been friends for 45 years and grew up together in Glen Ridge, New Jersey,” explained Mr. Waldner. “We went cross country together, got our first apartment together in Hollywood. We made films in high school and college.

“Bob had a career on the West Coast editing commercials for 20 years,” Mr. Waldner said. “He’s back in Jersey now, retired, not married, no dog, no house plants. So he’s got the time. This was all pro-bono and we had a budget of zero dollars.”

With no budget and no real possibility of return on investment, a logical question for Mr. Waldner is, why spend so much time and energy documenting a high school theater production?

“Mainly, because it’s so much fun and is such a positive program,” Mr. Waldner responded. “I’ve seen and heard stories first-hand about how this program touches kids lives in a profound way. It’s one of the few things where there are no egos. It’s all positive and lots of hard work, but fun.”

Having watched Mr. Kaasik, his wife, Anu, produce the shows, and the cast and crew pull off the musical every year, Mr. Waldner knew there would be plenty of great material to capture on tape. He approached his job like a fly on the wall, and did his best to remain a quiet observer throughout the process so the students wouldn’t be tempted to play to his camera.

“There’s always a moment every year where it looks like it’s going to hell. Usually its about a third of the way through — nothing’s working, the kids aren’t ready,” Mr. Waldner said. “I always find it funny, I see the beads of sweat on John’s head and every year it happens.”

And every year, the kids pull it off in the end.

Among the cast members that Mr. Waldner documents in the film is Julia Labrozzi, who graduated in 2017 and performed in eight musicals throughout her career at Shelter Island School.

“I did a rough cut at end of the school year and I said if any of you want to be a part of it, we can sit and talk today,” he said. “Five of my favorite kids — two juniors and three seniors — stayed. It was a sweet time and they all got really emotional. Julia said, ‘The first year Mr. Kaasik let me be this little witch and it meant so much to me, and he saw a future star.’”

Mr. Waldner noted that a benefit of being such a small school is the ability to cast younger middle school kids alongside the high school students they idolize.

“It’s a real team effort. Nothing negative is ever said,” he explained. “The older kids support the younger kids and take them under their wing. There were four sets of siblings in the show and I was so struck at how nice they were to each other.

“The film is about the connection between these kids. I could see this through the viewfinder every night,” Mr. Waldner said. “Programs like this give students personal one-on-one experiences. They’re working toward a common goal. Speaking, touching and dancing and not sitting in front of a screen. That’s why its so important to have programs like that.”

The enthusiasm and words of wisdom offered by Mr. Kaasik throughout the process — and his emphasis on modesty and teamwork — are some of the scenes Mr. Waldner likes best in the film.

“One of my favorite parts is where John’s talking about the curtain call. When you look at the self-centeredness in this country, it’s so refreshing to see the students taking that away with them,” Mr. Waldner said. “He tells them to come out on stage with humility and be team players. I knew those things would be a big part of the film.”

The film also offers introduces audiences to the many adult volunteers and supporters who help behind the scenes as choreographers, acting coaches, prop builders or costume wranglers. Among them was Jack Monaghan, a former teacher and coach who was a huge supporter of the school’s theater program. The film is dedicated to Mr. Monaghan, who died on September 6 at the age of 82.

“I wanted to capture the spirit of the play itself. The positive vibe and affection the kids have for the adults,” said Mr. Waldner. “It’s a great, successful program and why kids come back.”

“Behind Curtains” will be shown on Friday, November 24 at 7 p.m. in the school auditorium. Admission is free.