There was once a more mysterious version of celebrity. The wall that separated a famous artist’s performance from that same artist’s private life was more difficult to scale. Popular singers didn’t routinely executive produce advertorials stuffed with behind-the-scenes footage of themselves: no rehearsal time laid bare, no banal shopping trips, no nervous visit to their personal ear, nose, and throat specialist (to date, Justin Bieber has made two such films). Fans didn’t have the immediate access provided by the internet; a star was more or less allowed to keep the public at arm’s length.

Filmmaker Tom Volf reaches into the past to change that in “Maria by Callas,” a lovingly assembled documentary about the life and career of American opera legend Maria Callas, whose voice is considered to be among the greatest of the 20th century. Here, the private Callas is made public.

Curated from live performance footage, television interviews, the singer’s own private letters and unpublished personal writing (some of which are read in voiceover by mezzo-soprano Joyce DiDonato), as well as photos and memorabilia lent by friends and other archival sources, what emerges is more respectful portrait than painstaking biography, but one that will satisfy fans and undoubtedly draw in newcomers to the legacy of opera’s most notoriously expressive performer.

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The biographical details are included, to be sure: Callas describes her New York childhood with a demanding mother, and her entrance to a Greek conservatory at 13, where she studied under, and subsequently developed a lifelong devotion to, Spanish soprano Elvira de Hidalgo. Rather than explain her adult career, however, we get to see and hear Callas perform arias from works such as Verdi’s “La Traviata” and Bellini’s “Norma,” start-to-finish, thrillingly, uninterrupted.

The controversies each take their bow for the purposes of mild correction. Her creative battles with the Metropolitan Opera’s Rudolph Bing, bouts that led to her eventual firing (“Men don’t like the truth,” she says in one interview, and if it wasn’t referring to Bing specifically, it might just as well have been). Then there is the infamous 1958 cancellation of a gala performance in Milan of “Norma,” where she walked out after the first act, unable to continue due to bronchitis. These troubles are all defended in what was at one time private communication, a gentle rebuke to popular narratives of the “difficult” diva once known as “The Tigress.” (In one televised interview, she demurs when branded as “tempestuous.”)

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Then there is her long affair with Aristotle Onassis, one that facilitated her divorce from husband Giovanni Meneghini, and which was interrupted by Onassis’ marriage to the widowed Jacqueline Kennedy. Callas and Onassis resumed the romance in the middle of that marriage; Callas publicly referred to their liaison as “friendship” and privately described it as a relationship that “made [her] feel liberated” as her vocal power declined, her stage career cooled, and her desires to live a more traditional life — or at least as traditional as a life can be on a billionaire’s yacht — took over.

At the time a source of screaming tabloid ire, here the relationship takes on a wistful quality, Callas’ plea for comfort and joy that fame’s spotlight denied her, a break from what she called “the Callas I have to live up to” and one that makes the included performance of “Vissi d’arte, Vissi d’amore (I lived for art, I lived for love)” from “Tosca” all the more poignant and heartbreaking.

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If there is a glaring omission in “Maria by Callas,” it would have to be that of Callas’s uncompromising work ethic, the years of arduous process it took to become that name she had to live up to. It’s also probably an understandable void, a private element of her vocation that never went before cameras. In its stead, Volf peppers glorious recital footage with warm novelties like Callas talking about her recipe collection, footage of her being jostled by a baby elephant at a gala, of being whisked through airports in impeccable vintage outfits, of backstage costume preparations, and on-set footage from her role in the 1969 Pier Paolo Pasolini film “Medea.”

“Destiny is destiny, and there is no way out,” Callas matter-of-factly states in a TV interview, seemingly resigned to her fate as a special creation, explaining nothing more on the subject. And as a document of a special creation, “Maria by Callas” is very nearly enough, thanks in no small part to that generous helping of footage where she fulfills that very destiny. It’s a powerful reminder that private walls can stay put when she’s singing Bellini’s “Casta diva,” that the music is more than enough, that we can let the mystery be.