Here's a strange career arc for a gifted young film actor: Would you cast aside your promising future as a performer to become assistant to the most brilliant, perfectionist and demanding moviemaker of the 20th century?

An odd choice, surely, but one that Leon Vitali couldn't resist when the opportunity arose to become Stanley Kubrick's factotum. A popular British TV actor earlier in his career, Vitali said after seeing "A Clockwork Orange," "I will work for that man."

He reached the pinnacle in 1975 with what began as a small role in Kubrick's brilliant 18th-century aristocratic saga "Barry Lyndon." Playing Lord Bullingdon, the angry, frightened heir who faces a conflict with a bumpkin interloper stealing his birthright, the 25-year-old Vitali delivered the story's tragic conclusion, a scene that many consider the greatest dueling sequence in cinema history.

Kubrick wrote that climactic episode and several more featuring Vitali deep in the film's production because he saw Vitali delivering much more than a supporting role. Vitali endured 30 takes of a real on-screen beating from the film's star Ryan O'Neal, with Kubrick ordering O'Neal to hit harder.

The documentary "Filmworker" by Tony Zierra uses scores of clips from the classics, including abundant behind-the-scenes material, to appraise the quarter-century of Vitali's deep commitment to Kubrick, the genius who was already behind some of the greatest films ever made, from "Paths of Glory" to "Lolita," "Dr. Strangelove," "2001: A Space Odyssey" and "A Clockwork Orange." He stood alone with his pioneering advances in special effects and unconventional approaches to storytelling; who can say confidently what any genre-smashing Kubrick film is truly about? It's not uncommon for film scholars to attribute a 200 IQ to Kubrick. On the basis of Zierra's new film, that strikes me as a bit low.

So moved was Vitali by Kubrick's personal guidance and vision while acting on "Barry Lyndon" that he volunteered to became his assistant in any capacity available. The offer was accepted. With Kubrick's control-freak demand of precise, unlimited work done at all times every day of every week, decades of divine suffering followed.

As his own producer Kubrick was famously frugal, rewarding Vitali with a pittance of salary and rare moments of gregarious social contact. Stories of Kubrick driving some of his cast to the razor's edge of madness with hundreds of near-identical takes are common. "Filmworker" shows how he applied the same demands to his off-screen crew.

Vitali took various positions with the master. He served as Kubrick's secretary, sound artist, license negotiator, acting coach (he recruited and helped 4-year-old Danny Lloyd, who played young Danny Torrance in "The Shining," and created the unplanned roles of creepy twins who terrified Danny when he discovered sisters Lisa and Louise Burns).

He also discovered R. Lee Ermey, the unforgettable foul-mouthed Gunnery Sgt. Hartman in "Full Metal Jacket," from a real military barracks. Vitali was then sent to fire the performer who had been guaranteed the role. As Vitali says, it "was the not-so-fun part of the job."

He was a 16-hour-a-day visual consultant supervising precisely calibrated new prints for Kubrick's films. He worked in almost every capacity in "Eyes Wide Shut," returning to the screen as the masked orgy master Red Cloak.

Draining as the work was, Vitali used it all as an unlimited backstage pass to Kubrick's master class in creating movie masterpieces. The documentary mildly implies that his obsessive serfdom drove Vitali a bit mad, but it's a masochistic gesture that many a film geek can understand. If you could be Leonardo's scullery maid or Einstein's chauffeur, wouldn't you?

Vitali served as Kubrick's creative Jeeves and devoted Jack of all technical trades until the master's death in 1999 directly after finishing the final cut of "Eyes Wide Shut." It was not an ending easy to accept for a devotee who had difficulty perceiving his master as mortal, and it's clear in this film that Vitali still strongly feels the loss.

Beyond serving as the custodian of Kubrick's work, supervising preservation, restorations and rereleases, he has held onto the filmmaker's legacy in touchingly personal ways. When Kubrick's work became the focus of a major show at the Los Angeles LACMA art museum, Vitali, who lives in Southern California, was not offered an opportunity to curate the exhibition. So he quietly offered his time and expertise to guide groups through the legendary auteur's unique intersection of art and cinema.

"Filmworker" is partly a portrait of Vitali, who will turn 70 in July. But he largely, modestly stands aside, sharing his insights into the visionary artist he served. And if you want to know whether Kubrick really faked the Apollo 8 moon landing in 1968, there's no one more trustworthy to ask. No one on Earth, anyway.