We tend to think of film directors as generals, a cliché that’s useful, and accurate, as far as it goes. Yet compared to almost any other vocation, the essence of what it means to be a film director — especially if you’re a serious and powerful artist — is that you occupy a dozen roles at once. You’re a politician, an acting coach, a therapist, a budget manager, an image technician, a literary dramatist, a back-room manipulator, a dictator, and (when you need to be) everyone’s best friend. Not to mention the things that often go with the job: a media star, a sexual hound dog, and a workaholic.

When you see a typical documentary about a filmmaker, much of this stuff often ends up on the cutting-room floor. But Jane Magnusson’s “Bergman — A Year in a Life,” a portrait of Ingmar Bergman in the pivotal year of 1957 (though it covers his entire life and career), is one of the most honest and overflowing portraits of a film artist that I can remember seeing. It’s one of two Ingmar Bergman documentaries at Cannes this year (the other, which has yet to screen, is Margarethe von Trotta’s “Searching for Ingmar Bergman”), and it captures Bergman as the tender and prickly, effusive and demon-driven, tyrannical and half-crazy celebrity-genius he was: a man so consumed by work, and by his obsessive relationships with women, that he seemed to be carrying on three lives at once.

There are a few reasons why Magnusson chose 1957 as the lens through which to scrutinize Bergman. It was the year that he ascended to the iconic plateau of his creative power and fame — the year when he became the great Ingmar Bergman, the art-house superhero whose black-and-white images of torment and symbolism came to represent the lure of cinema in the second half of the 20th century.

Yet it was also a year when Bergman tore himself apart with work in a way that would define him. He’d shot “The Seventh Seal” the summer before (in a “forest” next to a dull apartment complex), and when that movie was released, in January, the image of Max von Sydow playing chess with Death made it the most iconic art film of its time. It gave Bergman carte blanche to write and direct the movies he wanted, on his own supremely personal and demanding terms. From that point on, says Magnusson, his films were always about himself. But you knew that, right? That’s what gives Bergman’s dialogue its sizzling confessional charge. Of course, in its “Madame Bovary, c’est moi” way, this also comes off as a bit of an oversimplification (he was each character in every film?), but we take the point.

Bergman had yet to even conceive of “Wild Strawberries,” but by the end of 1957 the film would be written, shot, and released. He would also make the television movie “Brink of Life” and direct four large-scale theatrical productions, the first of which was a massive staging of Ibsen’s “Peer Gynt,” a verse-play that many thought to be impossible to stage. Bergman turned it into a five-hour phantasmagorical adventure spectacle that made him the toast of Sweden.

At the same time, the 39-year-old Bergman was carrying on relationships with four women, one of whom was Bibi Andersson and one of whom was his wife; his three marriages has given him six children, whose birth years he could scarcely remember. It’s astounding that Bergman could find time in his schedule for stomach pain — but, in fact, he was stricken by it. He had ulcers that woke him in the middle of the night, so that he seldom slept past 4:30 a.m. Yet even the rebellion of his intestinal system served a purpose: He would go to the hospital, and once there he used the time to write.

It’s become routine for ambitious entertainers to put their stamp on a multi-tasking array of projects (he’s a rapper! and an actor! and a fashion mogul! and a we-are-the-world foundation organizer!). But Magnusson, who has already made one Ingmar Bergman documentary (“Trespassing Bergman”), directs and narrates “Bergman — A Year in a Life” as a psychoanalytical portrait of the artist. She doesn’t just chronicle the awesome draining fact of Bergman’s commitment to his work. She reveals how it was all about creating a bubble of alternative reality that he lived inside: a neurotic fairy tale that never had to end, and that paradoxically turned out to be the one place where he could be sincere. (The film never makes the connection between Bergman’s work insatiability and the number of people he had to support, though clearly that was part of it.)

In an interview clip, Bergman says that because he never stopped moving from one project to the next, he lived in an eternal “now.” What got left in the lurch, of course, were his children and his families — and, maybe, his own mental health. Then again, without work, the demons might only have come out more.

His temper was fearsome, and we see examples of it, thanks to some nicely edited montages of on-the-set rage. It explodes out of Bergman with a cobra-like quickness, when he wants silence or gets interrupted. The roots of his anger lay in his childhood, and it’s here that “A Year in a Life” offers a fascinating revisionist history. Bergman’s minister father was very much the punitive taskmaster he has described, but it was his older brother, Dag, who got the beatings; Ingmar was the golden boy. But he appropriated the abuse heaped upon Dag to embellish his own mythology — something that Dag was set to reveal in a TV interview in the ’80s that Bergman squelched. We see clips of it here, and they set up a disturbing personality syndrome: that Bergman lied whenever he felt like it, twisting reality to his own ends.

In the late ’50s and early ’60s, Bergman still looks like a mensch: the tall, skinny, slightly awkward loping figure who (like a lot of film directors) was almost handsome. He was happy on the set, enthused about the process, and in his trademark beret and leather jacket, with his forlorn yet forceful long-faced stare and jumbled-tooth grin, he projected his own cool/uncool Scandinavian mystique. He comes off as warm and funny (which only heightened the dour mystery of his aesthetic), and by the time he’s interviewed by Dick Cavett in the early ’70s, it’s Cavett who’s nervous and Bergman who seems, in his modesty, like a superstar.

The film treats Bergman’s serial relationships as a form of erotomania. Liv Ullmann is interviewed, and there’s a funny moment when she can’t remember which transcendently beautiful actress Bergman slept with in what order. But Ullmann, through tears, also claims that Bergman “was the most ordinary, everyday man that you could ever live with.” That was his off-set demeanor. Shooting movies, he was a control freak who ate nothing but yogurt and biscuits (and flew into a tantrum if you took one of them), and he wasn’t above pulling a monstrous stunt like having Gunnar Björnstrand’s doctor, during the filming of “Winter Light,” give Björnstrand a (fake) serious diagnosis, which depressed the actor into having an authentic spirit of gloom.

For a movie rooted in one period, “A Year in a Life” touches on everything from the earthquake that was “Persona” to the power struggle that consumed Bergman in 1995, when he restaged “The Misanthrope” at the Royal Dramatic Theatre and wound up in an ugly clash with his lead actor, Thorsten Flinck, whom he experienced as a rival. Flinck was no threat to him, but this was really Bergman fighting old age, asserting the force that he felt was draining out of him.

Yet in 1957, all of that force came together. It was about Bergman’s hunger and genius, his insane lust for life, the stories he needed to tell, and about something else — a moment in the 20th century when a great many people got hooked on movies that turned the darkness of our hidden hearts into drama that scalded and cleansed you.