For anyone who has ever lived backstage, the title Don’t Say Good Luck is instantly recognizable. You do not wish someone “good luck” before a performance. It is bad luck. In theater, people say “break a leg”; in dance, opera, and musicals, each world has its own variation, but the superstition cuts across generations. It is one of those unwritten rules performers seem to learn almost before they learn where the wings are.

Only Murders in the Building has already had great fun with the world of theatrical superstitions, but Don’t Say Good Luck begins with one of them and moves into something much less mysterious and far more intimate: that age when everything seems to be beginning just as something very important is coming to an end.

Sophie Birenbaum, played by Sunny Sandler, lands the lead in her school production of Waitress. It should be one of the great moments of her adolescence. At the same time, she learns that her mother Elizabeth’s cancer has returned. Melanie Lynskey plays her mother, Max Greenfield her father, while Stephanie Beatriz, Bebe Neuwirth, Steve Buscemi, Jack Champion, Scarlett Estevez and Jon Lovitz round out the cast. Julia Hart directs and co-wrote the screenplay with Jordan Horowitz, based on an original script by Laura Hankin.

The story is rooted in Hankin’s own life. As a teenager, she landed the lead in her school musical while her mother was facing terminal cancer. In real life, the show was Moby Dick! The Musical; in the film, it became Waitress. The change is particularly inspired because Waitress is itself a story about a woman trying to understand who she is while her life shifts around her.

The musical grew out of Adrienne Shelly’s small 2007 independent film and found a second life on Broadway through Sara Bareilles’ music and lyrics. Among its songs is “She Used to Be Mine,” one of the most emotionally brutal — and beautiful — songs written for musical theater in recent years. And that brings us to Sunny Sandler.

Yes, she is Adam Sandler’s daughter. And I see no reason to pretend that this is some kind of scandal. Adam Sandler has been casting family members in his films for years. Sunny started appearing in small roles when she was still a child and later had a much bigger part in You Are So Not Invited to My Bat Mitzvah, also for Netflix, alongside her sister Sadie. Adam produces Don’t Say Good Luck, as does Jackie Sandler, Sunny’s mother. None of this is hidden.

It is also worth remembering that Sandler was one of the first major Hollywood stars to bet seriously on Netflix. Back in 2014, when the platform was still trying to convince Hollywood that it could also become a movie studio, he signed a deal for four exclusive films. The partnership was widely mocked at the time. It was renewed again and again, and Sandler ultimately proved something even his critics have never been able to deny: he delivers an audience.

He remains one of Hollywood’s strangest comedy phenomena. His films can trigger endless arguments about quality, but he knows his audience, has an extraordinary instinct for entertainment and has always had a much better musical ear than his own voice might suggest. Because that is one of the delightful ironies of Don’t Say Good Luck: Adam Sandler loves music.

Music runs through his films, his comedies and his characters. Just think of The Wedding Singer. His soundtracks are often excellent, and he has always joked about singing with that small, slightly crooked voice that fits perfectly into his comic persona. And then his daughter opens her mouth. Sunny Sandler can sing spectacularly well.

Not “well for an actress.” Not “well for a teenager.” She has a powerful, musical voice, real projection, and something that immediately evokes Broadway. The moment she starts singing, the film seems to grow along with her.

As an actress, she still has room to grow. Sunny is very young, and it would be unfair to demand a dramatic range that may simply come with experience. There are moments when she still lacks layers, especially because Sophie is moving through enormous emotions: sexual awakening, first love, changing friendships and, above all, the first real encounter with a definitive goodbye.

Physically, there are moments when she reminds me of Sofia Coppola — and I am thinking specifically of Sofia Coppola in The Godfather Part III, which is not necessarily a compliment to the acting. There is a certain stiffness, a still-visible difficulty in allowing very different emotions to emerge effortlessly. Even so, Sunny carries the film. And when she sings, every hesitation disappears.

It is as though music gives her a freedom she has not yet entirely found as an actress. The phrasing, power and confidence are all there. At certain moments, you almost want the film to stop making her talk and simply let the girl sing for the rest of it.

The material helps. Alongside Waitress, the film plays with other pop references, including Katy Perry, and Julia Hart understands that music cannot function merely as decoration. It becomes the language through which Sophie is finally able to express what she does not yet know how to say.

That may be Don’t Say Good Luck’s greatest strength. We have seen this story countless times before. A teenager grows up while someone she loves is dying. First romance. Friendships changing. A family trying to remain whole. The impossible balance between allowing yourself to feel happiness while someone beside you is suffering.

None of it is new. But the film does not need to be new to work. It needs to be delicate. And it is. Melanie Lynskey helps enormously. Elizabeth could easily have become that familiar movie mother who exists only to die and force the protagonist to grow up. She does not. There is humor, irritation, fear, motherhood, and a very recognizable attempt to keep being a mother even as her own body begins imposing new limits. The film is really dealing with grief before death — anticipatory grief, that strange experience of beginning to lose someone who is still there.

Sophie, meanwhile, has to learn something even harder: how to keep living while loving someone who is dying. That is why Waitress works so well inside this story. That is why “She Used to Be Mine” works. And that is why Sunny Sandler works.

We may not yet know what kind of actress she will become. But we already know what kind of singer she is. And after Don’t Say Good Luck, I am very curious to see what she does with that voice.