When you watch a nonfiction film you expect there to be captions and subtitles to help you identify the talking heads. In Les Blank’s A Poem Is a Naked Person—the centerpiece of the Brattle Theatre’s Reel Music Film Festival, which runs through next week—there are only a brief few words on screen. And when those bits of text show up, they’re typically meant to clarify a phrase heard in passing, not a face seen within the frame. So sentences like “this-that’r the other” appear onscreen, bolded like large-font headlines. The ostensible subject of this movie, shot between ’72 and ’74, is the legendary session artist and country songwriter Leon Russell. Blank, who passed in 2013, approached his topic the way a gonzo journalist might have: with an eye that wanders, and an ear that’s attuned to funny phrases.

Blank was blessed to inherit this subject for his first feature-length film: you couldn’t invent a better headliner for a ’70s time capsule than Russell. (Though completed soon after shooting, Russell blocked the movie’s release until this year, for reasons become clear as you watch it.) He’s got the Gandalf beard to hide his face—it was Nashville in the early 70s, man—and the words coming through the scruff have a THC-infused, post-revolution slowness to them. And his hands have the animated movement of a Donald Sutherland character; they’re always bopping and bouncing outside of Blank’s 4:3 frame. But of course the hippie-sway and the zoned-out diction percolate into something transfixing and poetic when the man is onstage. And in those moments Russell can look like a saint off of a Renaissance painting. He’s got the essential elements for being a disreputable messiah: grotesquerie and grace.

If this were a concert film, that dichotomy might be the whole point. But this isn’t a concert film. Naked Person follows Russell and his crew through Tulsa and Tennessee. And the editing cuts out of performances mid-song, just to allow for jokes and testimonials from the locals on the margins. There’s a quote in the end credits from Jean-Luc Godard, saying that the day of the director is dead. But Blank disproves the quotation himself. Russell, who’s also the producer and financier, gets sidelined. And instead we look at whatever catches the director’s eye. There’s a pair of Oklahoma octogenarians excited by this local boy and his hippie-adjacent hair; there’s a painter named Jim Franklin filling up an empty pool with psychedelic iconography. At one point we’re just watching two sets of water ripples as they slowly begin to intersect, like clasping hands.  This is a tourist’s home movies, a southern-city symphony, a stoner travelogue, a time capsule. And it all moves the way that Russell’s hands do: wherever Blank goes, he goes without warning.

Sometimes ideas are hidden between those images: When Blank cuts between Russell’s spirited performances and a righteously orating preacher at an African-American church, you know what tradition the filmmaker thinks his subject belongs to. And when he closes with a performance of “Amazing Grace,” you have your interpretation confirmed. But Poem isn’t all that intellectual, generally speaking. Blank includes footage of a Tulsa hotel being destroyed. There’s a crowd of locals out, because they want to see the building one last time. Blank’s discursive craftsmanship accomplishes something similar. He catches us a glimpse of a revolutionized Southern rock culture: post-’Nam and pre-Reagan, imbued with rural tradition of bystanders, but energized by the urban intoxicants of rock stars. When that implosion happens, Blank intercuts it with two predatory acts—one involves a snake, and the other involves some cops. By now, all that’s left of the culture that entranced him is this movie itself.

A POEM IS A NAKED PERSON. BRATTLE THEATRE. 40 BRATTLE ST., CAMBRIDGE. FRI 11.13, 7PM. ALSO SCREENS SAT 11.14 AT 1:30 AND 3:30PM. NOT RATED.