If you’ve ever wondered just how co-dependent the relationship between filmmaker and subject can be in the course of filming a documentary, “Left on Purpose” answers that question. And how.

Documentarian Justin Schein harbored a lifelong interest in the Yippies –the  Youth International Party — pranksters and protesters in the 1960s and early 70s.

So he set out to do a movie about Mayer Vishner, a Greenwich Village fixture and key figure in the group that Abbie Hoffman and Jerry Rubin fronted. Vishner was a fresh-faced junior member of the top tier of the Yippies.

“I wasn’t just hanging out with giants,” he explains, on camera. “I was helping them BE giants.”

An anti-Vietnam War rally organizer, public speaker and later alternative press reporter, critic and editor (of LA Weekly), Vishner seems, even as we meet him in his 60s, like a man who affects change. There’s a neighborhood veggie garden he managed to start, later in life, in his little corner of Greenwich Village.

But Schein points his camera at a 60something hoarder, a not-quite-shut-in who lives off disability payments and family charity. He wanders his cluttered apartment in his underwear and assorted T-shirts, plays with his cat and watches TV.

And he and drinks. A lot. Ironically, the old leftist drinks giant bottles of Coors Light, the product of right wing oligarch Adolph Coors.

About 30 minutes into the movie, he lets Schein know, on camera, that “I’m dying of loneliness…I don’t want to do this any more.” He’s not talking about the movie, which he regards as his autobiography. He wants his “final political act” to reflect his existential angst. He wants to die on camera.

Schein is rattled and over the last hour of “Left on Purpose” he tries to find a way out — bringing Vishner to family and old friends who might talk him out of suicide, filming Vishner’s doctor and others refusing to help him end his life.

“I can’t just stay behind the camera and watch him kill himself.”

Schein’s touching film has him and his wife Eden Wormfeld wrestle over the increasingly needy Vishner’s demands on Schein’s time, just as they’re having a baby. Vishner comes off  as clinically depressed (he was once in a psychotherapist’s care) self-absorbed, stubborn, smart and resolute.

“It’s time to go.”

Those closest to him, those he hasn’t pushed away, see the problem clearly. It’s the wall of empty beer bottles, the never-idle-long bong, that sapped his will to live, or even to organize the treasure trove of ’60s-70s radical life that he’s saved but never archived.

Will Vishner go through with it? Will Schein stick around to the end?

Either way, Schein has captured a life story worth remembering and the pursuit of a death with purpose. And he’s probably learned a valuable lesson about journalistic distance, when to maintain it and be just “the observer,” and when to give in to your humanity when you see suffering right in front of you.

MPAA Rating: unrated, with adult subject matter, profanity

Cast: Mayer Vishner, Andrew Hoffman, Michael Ventura, Justin Schein, Eden Wurmfeld

Credits:Directed by Justin Schein,  David Mehlman. A FilmBuff release.

Running time: 1:25