NEW YORK - At the end of Cameron Crowe's moving and eloquently simple documentary, "The Union," Elton John sings "You're never too old to hold somebody."

That lyric is entirely appropriate for a film that is one warm, extended embrace from the music legend to his idol, Leon Russell.

This chronicle of the making of John and Russell's 2010 Universal album of the same name is also a valentine from a filmmaker for whom music has always been an indispensable element of his movies. Reinforcing that connection, John began his live performance following the Tribeca Film Festival's opening-night screening with "Tiny Dancer," a song used to stirring effect in Crowe's 2000 feature, "Almost Famous."

The most visible directorial touch here is the split-screen employed to show the two musicians on opposite sides of a studio, or to juxtapose present-day images of them with their 1970s high style. Otherwise, Crowe's work is anything but intrusive. You get the sense he counts himself lucky just to be in the same room while these guys work. That congenial tone might make "The Union" a little reverential for non-fans, but it should find an eager audience of devotees on TV and DVD.

John toured with Russell back in the '70s, but the two had not seen each other in 38 years when they met again in Los Angeles to begin work on the album. John conceived the project as a tribute to a piano man and songwriter who was a major influence on him; his aim was to recapture the sound of Russell's vintage releases.

Produced by T-Bone Burnett, the result was listed by Rolling Stone among the top five albums of last year. It merges the expansive flavors of Russell's music -- combining rock 'n' roll, gospel, soul, blues, country -- with enveloping narratives and soaring sounds that evoke the golden years of John's songwriting collaboration with Bernie Taupin, another contributor to this album.

A 2011 inductee into the Rock & Roll Hall of Fame, Russell had drifted into semi-obscurity before the album was made. At times he shows the watchful timidity of a hermit lured back into society. When John attempts to high-five him soon after their reunion, he says, "I don't know how to do that. That's some kind of sports thing, isn't it?" There's also a dry, self-effacing quality to his humor, and a notable lack of ego.

While his outfits might be less outre, Russell hasn't significantly altered his look in the four decades since he was heading Joe Cocker's band on the "Mad Dogs and Englishmen" tour. But while his curtain of hair, epic beard, spectacles and occasional top hat back then gave him the air of an intimidating hippie wizard, he's now a more benign, white-maned figure, accurately described by John as looking like God.

Both artists purveyed different brands of flamboyance back in their hey-days, illustrated by some fun archival footage. Seeing John in his red hot pants or Donald Duck costume never gets old. Crowe provides a brisk account of the incredible sweep of Russell's influence in a montage of hit songs from the '60s and '70s on which he played as a session musician.

While there's no attempt to create artificial tension in what's basically a love letter, the film acknowledges an interruption in the creative process as Russell underwent emergency brain surgery. His gradual recovery appears to be fueled by the music, peaking when some soulful backup singers enter the studio and start shoop-shooping, which has Russell stroking his beard with pleasure.

The tenderness John shows his collaborator is clearly genuine. Watching him overcome by emotion as Russell, not long out of hospital, sits at the piano and performs the gravelly hymn "In the Hands of Angels" for the first time, John seems less a music giant than a man acknowledging an enormous debt of gratitude.

Famous faces stop by during the writing and recording process: Booker T. Jones plays on one track, Brian Wilson sings harmonies, Stevie Nicks drops in and recalls opening for Russell with Lindsey Buckingham a few years before they formed Fleetwood Mac. But the film is above all a gesture from one musician to another, a heartfelt testament to the rewards of collaboration, and for John, an act of humble fandom.

"The Union" is dedicated to Reginald Dwight and Claude Russell Bridges, the birth names of its two subjects. That choice is fitting for a portrait that looks beyond the fame of either artist to provide intimate access to them as they return to their roots.