FANS tend to remember their first exposure to the strange world of Daniel Johnston. For many UK indie kids of the 1990s, the cue came from another troubled American musician, Kurt Cobain, who wore a T-shirt promoting Johnston's Hi HowAre You album in countless photo shoots circa 1992. Further research (let's not forget, in pre-download days) led the faithful to a shambling thirty-something singer/songwriter and cartoonist who had recorded the bulk of his fragile oeuvre on to cassette tapes in his parents' basement.

Closer examination revealed a restlessly creative individual whose playful yet rawly confessional songs were the legacy of a lifetime of painfully unpredictable mental health. It was a bleakly romantic personal history that fitted the culture of the time; yet the strangeness of Johnston's work and the frailty of his emotional state meant that the megastardom of which he dreamed would remain perpetually elusive.

For Jeff Feuerzeig, the director of the new film The Devil and Daniel Johnston, exposure to the man and his music came via the hotbed of discovery that was 1980s American college radio. "It was a small underground world in those days, before the internet, " Feuerzeig recalls. "It was just a few hundred people here and there; and it was regional."

Feuerzeig was at Trenton State College, New Jersey; Johnston was living in Austin, Texas (and working at McDonald's, where he would take calls from record company execs). It was a 1990 broadcast on New Jersey's WFMU radio that brought them together. "Daniel produced a one-hour special, which is legendary, " Feuerzeig says. "He interviewed himself in multiple voices, he was performing skits, he was improvising songs - and he led everybody to believe he was broadcasting from a mental home. He took calls from the audience; I phoned in, so that was how we met."

Johnston's career began to flourish as the pair became friends; but a major deal with Atlantic Records fell apart when Johnston experienced a severe bout of depression. Daniel moved home to live with his parents. The film covers this period in detail; but it's ultimately the story of an improbable survivor, whose creativity and liveliness endure against the odds. Feuerzeig had been contemplating a movie since 1990, but the idea crystallised when Johnston recovered sufficiently to cement his status as a cult musician with tours and new recordings. "I watched him play a sold-out show at (NewYork's) The Knitting Factory in 2000, and I called my producer Henry Rosenthal and said, 'Dan Johnston has a third act. He lives. We should make this movie now'."

And how did Johnston feel about being immortalised? "It didn't take any persuasion, " says Feuerzeig. "Daniel's been making his own movie . . . he's been documenting himself his whole life."Among the many strange elements in the film are Johnston's own adolescent home movies, which document his battles with his mother and his unrequited love for a schoolmate named Laurie. "They're the best home movies I've seen, " Feuerzeig says.

The Devil and Daniel Johnston provides a sensitive account of mental illness. Johnston remains an enigma throughout; his devoted parents, his famous friends and the filmmakers alike are always at arm's length. "To quote Dan's dad, we were all pawns in the theatre of Daniel's mind, " says Feuerzeig. "Daniel was the ultimate self-promoter; he exploited his own mental illness. While the creation of his art is incredibly pure, the presentation is all preconceived. He's hardly an idiot savant; he's always the smartest guy in the room."

It's this Daniel Johnston who emerges in the film: a fantastically complex artist who created and lived his own myth. Feuerzeig says: "People have to rethink Daniel Johnston - to see him as something more than just this lo-fi guy."

The Devil and Daniel Johnston opens on Friday at GFT, Glasgow.