It's a story well known to Robbie Robertson fans and those who read his 2016 memoir Testimony: A 16-year-old Robertson convinces Ronnie Hawkins to let him join his band with the promise that "no one will work harder."

(The Hawk, so the story goes, nodded and said, "I know.")

Some 60 years later, a Canadian filmmaker in his early 20s approached Robertson in hopes of directing a film based on that memoir, using similar words while lobbying for the job.

"I was jumping up and down, being like, 'It's gotta be me! No one else will give this more than I will,'" said Toronto director Daniel Roher, now 26.

"I recognized that feeling," said Robertson. But he said not all involved with the documentary initially thought it should be put in the hands of such a young filmmaker.

"I've been through so many things that I have done that they weren't sure [of], and then I made it happen," Robertson said. "That just gave me the confidence and feeling of, Go get 'em, kid!"

Robertson, left, and director Daniel Roher, right, are shown on the set of Once Were Brothers. (Piper Ferguson)

The product of that collaboration — a film called Once Were Brothers: Robbie Robertson and the Band — will now open the Toronto International Film Festival on Thursday.

Its opening night gala status speaks to the rising cachet of music documentaries.

The likes of The Last Waltz, the Martin Scorsese documentary about The Band's last performance, were once rare. But with the increasing number of platforms where these films can air — from Netflix to specialty channels to film festivals — rock docs are booming.

Watch: Robbie Robertson reacts to the new documentary about his legendary rock group The Band

Canadian musician Robbie Robertson was surprised by his own emotional reaction to reliving his time with The Band in a new documentary called Once Were Brothers. 2:32

While these films may promise an all-access, unfiltered view of a star, not all deliver on that.

Stars and their management wield increasing power over these documentaries, often producing or funding them themselves, leading to questions of who controls the narrative that the audience is seeing.

Record labels making documentaries

Perhaps more so than any other celebrity-based documentary, music docs live and die by the access the filmmaker is granted — and, of course, by featuring the artists' best-known songs.

Getting the rights to those songs is usually not possible without the permission of the artists or their labels, which is why the co-operation of a subject is often crucial to a music documentary, compared to other profile projects.

But record labels have recently become active participants in seeking out documentary projects that profile the artists on their rosters. In addition to facilitating access to the star and archival footage, many labels have launched documentary film arms that help fund and distribute these docs.

BMG Music's film arm, founded in 2015, recently produced and funded high-production documentaries on David Crosby and Joan Jett — both BMG artists.

While the labels often won't spell it out, the incentive is clearly financial: A popular documentary has a power to boost music sales.

In a recent Billboard article, Why the music industry is betting on biopics and documentaries for its next revenue boost, journalist Claudia Rosenbaum crunched the numbers to show how a popular biopic — like Bohemian Rhapsody — or a popular documentary — like Amy, about Amy Winehouse — can drive up the sales of an artist's back catalogue or increase streaming numbers.

But that doesn't necessarily mean all of these documentaries are fawning "puff pieces."

Amy, directed by acclaimed documentarian Asif Kapadia, was produced by Winehouse's label, Universal Music Group. It angered her father, Mitch Winehouse, by portraying him as an absentee parent and, later, an enabler of his daughter's self-destructive behaviour. It was also the most-watched documentary of 2015 and won an Oscar the following year.

David Crosby: Remember My Name received very positive reviews and a strong response at Sundance Film Festival, where it premiered earlier this year. In it, Crosby openly details drug use and the fact that most of the "guys he played with don't even talk to him."

Creative control vs. access

Still, financial or creative involvement of the star or the record label can certainly make things trickier. As a New Yorker piece criticizing Lady Gaga's Gaga: Five Foot Two documentary argues, "the resulting film often either feels like unapologetic hagiography or is revealing only in extraordinarily calculating ways."

Canadian filmmaker Barry Avrich has cut his reputation on unflinching, often-unflattering docs about powerful men, from The Last Mogul about Hollywood power broker Lew Wasserman, to his unauthorized documentary on Harvey Weinstein in 2011.

"If you sell your soul to the devil early on, that's going to be your reputation: gun for hire," Avrich said of the dangers, especially for young filmmakers, of working on docs where the star is heavily involved.

"If you're a documentary filmmaker and you don't have the wherewithal to deal with the egos and the machinations of dealing with talent management, then you're dead. Pick another topic."

Director Barry Avrich, at left, talks to David Foster, at the piano, on the set of his documentary David Foster: Off the Record. (Melbar Entertainment/Barry Avrich )

With that mindset, Avrich took on his most recent subject: Canadian musician and music producer David Foster. David Foster: Off the Record is also premiering at TIFF this year.

Avrich expected some pushback when dealing with the famously controlling Foster, known for his airtight production work with Céline Dion or Barbra Streisand. But he says his subject — who did not have a financial stake in the doc — didn't try to push his own agenda.

"Nothing was off limits," said Avrich, who talked to Foster's children and collaborators.

He said he hopes the resulting film is a fair portrayal. "I didn't go into the process saying, 'Let me try to find dirt on David.' I wanted to make a film about somebody that I don't think enough people know about."

'His version of the story'

As he readies himself to walk his first TIFF red carpet, Daniel Roher feels content that Once Were Brothers is very much his "baby" — even if Robertson's powerful friends, including Scorsese and Universal Music Canada, are among the producers.

"I was always very empowered by that, the respect I was given and felt by all my collaborators," said Roher. But he said he did take into account some suggestions made by Scorsese or Ron Howard, given their extensive experience.

Partially based on his 2016 memoir Testimony, Once Were Brothers: Robbie Robertson and The Band follows Robertson's early life through to the creation of the influential roots-rock group. (David Gahr)

If the documentary is not a "warts and all" exposé of Robertson, or The Band's interpersonal dynamics, or the oft-mythologized, decades-long acrimony between Robertson and The Band's lead singer Levon Helm, it's because Roher never intended it to be.

His film, he stresses, was inspired by and based on Robertson's memoir. "This is a film about The Band through the eyes of Robbie Robertson. This is his version of the story."

In the end, Roher said he hopes the audience sees what he saw: An inspirational story of how Robertson, a half-Indigenous, half-Jewish kid from working-class Toronto, pushed himself to the very top of the music and entertainment worlds through sheer tenacity and talent.

"What Robbie's life really embodies is that we have the agency and power in our own lives to invent ourselves, to be like, 'This is who I want to be. This is what I want to do.' And it's just a question of having the ability to go out and make it happen," he said. "And that's what Robbie did."