A documentary by Treva Wurmfeld. Unrated. Opens Friday, March 8, at the Cinematheque

Despite the ubiquity of buddy movies, sincere male friendships are rarely portrayed in depth on-screen, making Shepard & Dark remarkable even apart from the rock-star glow of its participants. Well, one of its participants.

Actor, playwright, screenwriter, cowboy, songsmith, and all-around handsome man Sam Shepard, who turns 70 this year, has been corresponding with nebbishy, pot-smoking Johnny Dark for more than 40 years. Both were bullied by scholarly, alcoholic fathers and are mordantly funny writers but otherwise couldn’t be more different. While Shepard went on to 12 kinds of fame, Dark—who could be played today by Dustin Hoffman—ended up alone in a small-town New Mexico bungalow, working part-time at a deli counter.

The donation of their letters, which Dark meticulously collected, to the University of Texas prompted filmmaker Treva Wurmfeld’s constantly surprising documentary about this unlikely palship. Despite some nice archival material, the luminously shot doc doesn’t get into Shepard’s basic CV. Briefly, he collaborated with the likes of Robert Altman, Patti Smith, Michelangelo Antonioni, and Bob Dylan, whose songs appear frequently here. This theatre-world lion won numerous Obie Awards and a Pulitzer for much-performed plays like Fool for Love and Buried Child. And among many film roles, he nabbed an Oscar nomination for playing Chuck Yeager in The Right Stuff.

There’s a lot of test pilot in Shepard, despite his actual fear of flying, and you can imagine what that’s like to live with. No one knows better than Dark, long ago married to an older woman whose grown daughter became his best friend’s first wife. Okay, stay with me now: when Shepard left her for Jessica Lange, Dark ended up raising Sam’s young son, notably absent here. That’s a whole lot of history for any two men to carry. Without spoiling anything, I’ll just observe that fame seems to offer no guarantee against despair, and money certainly can’t buy self-knowledge.