Adrienne Shelly was an actor, writer, director; a doting mother, loyal wife, vibrant friend; a committed artist who grew from performance to playwriting to film, finally coming into creative bloom when her life was tragically, brutally cut short at age 40. She died mere weeks before the indie film she wrote and directed, Waitress, got accepted to the Sundance film festival; the romantic dramedy starring Keri Russell went on to become a sleeper hit and spawned the hit Broadway musical of the same name, the first with an all-female principal creative team.

Shelly never got to see it; on the afternoon of 1 November 2006, she was strangled to death by a 19-year-old construction worker named Diego Pillco, after she walked in during his attempted robbery of the West Village apartment she used as her office. Pillco staged her body like a suicide, and she was found hours later by her husband, Andy Ostroy. If it weren’t for Ostroy’s doggedness in disputing the initial suicide ruling, and the identification of Pillco’s shoeprint in the dust of Shelly’s bathroom, the killing would have probably gone unknown and unsolved.

The grim details of Shelly’s death are handled bluntly, upfront, in Adrienne, a new HBO documentary directed by Ostroy that is part loving remembrance to a lost wife, mother and relentlessly creative soul, and part exploration of navigating unfathomable grief with a small child – Shelly and Ostroy’s daughter Sophie was just shy of three when her mother died. The 100-minute film opens with home footage of a buoyant Shelly at a Halloween party with Sophie the night before her death, then cuts to news coverage that presumed suicide, an assumption Ostroy immediately rejected.

From the day she died, Ostroy says, he was determined to find out what happened to Shelly, an early muse of indie film-maker Hal Hartley and later posthumous Sundance darling, and to make sure she was never forgotten. “It was really just a desire to humanize Adrienne, bring her back to life and show the world that she’s more than just a murder victim,” Ostroy told the Guardian of the film, which he began in earnest several years ago. “That she was a wife and a mother and a daughter and a sister and a friend.”

Ostroy, a first-time director, set out to answer three questions: who was Adrienne Shelly, what really happened the day she died, and what was it like for a family to live through unthinkable tragedy?

“The vision for the film was always very simple: life, death, aftermath,” he said. Life was clear – Ostroy kept a trove of home videos, on top of the many clips from Shelly’s films, starting with her debut as the lead opposite Robert Burke in Hartley’s The Unbelievable Truth in 1989. Born Adrienne Levine in 1966 to Elaine Langbaum and Sheldon “Shelly” Levine, Shelly, raised on Long Island, was a natural and enthusiastic performer. She performed early and often in school plays, and studied film production in college before dropping out a year early to pursue a career in Hollywood. (Her stage surname honored her father, who died suddenly of a heart attack when she was 12, a searing loss that resurfaces frequently in the film.) Blond, petite, doe-eyed, disarmingly emotive, Shelly found quick success along with Hartley, covering magazines and booking numerous underwritten roles.

By her late 20s, Shelly had grown jaded with the expectations and rampant sexism in Hollywood. In old videos, she talks about the pressures of the industry to “be fuckable”, to say yes to everything, to stay hot or relevant. “I think I spent a lot of my 20s as if I was in some dark adventure of some kind, not really, really feeling myself alive, and I feel like a real desire to not live my life that way any more,” she tells a camera on her 30th birthday in 1996. She helped found a theater company and started writing her own material, staging her first short play in 1993. The film’s survey of her personal archives recount Shelly’s greatest desires: to find a life partner (she met Ostroy through a dating site in 2001) and to become a mother (Sophie was born in 2003). Waitress, written when Shelly was pregnant, was her creative darling, the culmination of years of work and frustration.

Numerous friends from her childhood through to the set of Waitress recall Shelly as ebullient, remarkable, a unicorn, an alien, an old woman in a tiny beautiful body, special – “Adrienne was like no one else you’d ever meet in your life,” said Ostroy, echoing similar sentiments from Keri Russell, Paul Rudd, Cheryl Hines, Nathan Fillion, Burke, her mother and brothers, and many others. “When you were in her presence, you really felt, ‘Wow.’”

Ostroy weaves the initial mystery, and fundamental unbelievability of her death throughout, walking through his memories of the discovery and the police investigation that followed. Late in the film, at an unspecified date in 2019, he visits the Coxsackie correctional facility in New York, where Pillco was sentenced to 25 years in prison, without parole, for manslaughter. Over several minutes, Pillco, through a translator, recounts the steps of Shelly’s death; Ostroy shares photos of Shelly, of Sophie’s childhood and teenage years, memories her mother wasn’t present for. “The point of that interview was to get the truth of what happened that day, which I did get,” Ostroy said.

“And also to humanize Adrienne for him, so that rather than have a lasting image in his head of this panicked woman racing toward him calling out for the police, he sees a mother, a wife, a daughter, a sister. That he understood all the moments in the last 14 years that Adrienne missed, that Sophie missed, that I missed, that everyone missed.

“And maybe when he goes to bed at night and lies his head down and stares at the ceiling, he thinks about that, and it makes him feel something that he didn’t feel before.”

The meeting with Pillco is startling in its significance – Ostroy says he has no intention of ever communicating with him again – and its candor, but perhaps the most moving part of Adrienne is the film’s account of the aftermath: parenting through grief, explaining loss to a toddler, Sophie learning details of a person she barely knew in real life. The film almost serves as a manual for how to explain the inexplicable to a young child: in stark animation sequences, Ostroy reads diary entries recounting conversations from Sophie’s youth, ages two, three, four and seven, as she asks where mommy went, what happened to mommy, will they ever talk to the man who killed her, what happens when someone dies.

Sophie, now an 18-year-old senior in high school, also participates in interviews with her father starting at 15. “One of the beautiful parts of the film is that I had conversations with her and got real emotion from her that I don’t think I would’ve gotten if there wasn’t a camera there,” he said. Prescribed questions, for a project about her mother, allowed for a level of vulnerability an ordinary conversation might not. She starts the film admitting to her father that she hadn’t cried about her mother in years. By the end, the missing feels not abstract but palpable, and the tears begin to flow.

Grief is ongoing, fluid, non-linear; so is remembering. Adrienne is a film to honor a film-maker with so much left untapped, a woman whose name is on Broadway marquees yet unknown to the many fans of the film and show. The film cements her legacy “as an actor, as a writer, as a film-maker, as a creative force, as someone who had so much left to do”, said Ostroy. “I wanted people to fall in love with her, I wanted them to mourn and grieve her loss the way we do, and to feel that absence.”

• Adrienne is available on HBO from 1 December and in the UK at a later date