Oh Lord. How did I get here? I’m sitting on the floor in a corridor of an East London hotel helping a big, friendly manic-depressive put ketchup and salt on his lunch.

By now, we’re running over an hour behind schedule. Worse still, we’ve been wandering around and I’m not sure if his people will be able to find us.

This is all wrong. I should be asking this guy about his new album or his new art exhibition or the new documentary on his life, but it’s not easy in the circumstances. It was always destined to be an irregular experience, but interviewing alt-folk legend Daniel Johnston is turning out to be something of a babysitting assignment.

In person, the great polymath is every bit as childlike as the eerie waif vocals in his recordings. Doughy, with a beach-ball lithium paunch and severe tremors from years of psychoactive medications, he’s an affectingly fragile presence. At 45, Daniel is still cared for by his devoted elderly parents at their home in Waller, Texas. On more than one occasion, he’ll allude to something, then immediately ask me not to print it in case it makes his dad mad. For all his past troubles, he’s jollier than one might suppose.

“Are we going shopping now? Let’s go shopping!” he exclaims 40 seconds after I’ve turned on my recorder.

Yep. Daniel can be fun to be around but his attention span puts one in mind of a particularly hyperactive Tex Avery cartoon. I’ve read previous aborted interviews where, fixated on a crumpled piece of paper on the floor, he has simply forgotten there is another person in the room. Sure enough, he’s like a big kid. Although he answers all questions, you have to work pretty hard to hold his attention.

He slurps his coke and belches, then suddenly looks up from his burger and smiles.

“This meat is really great,” he gushes. “This may be the best meal I’ve ever eaten.”

I smile back, though really I’m biting hard on the inside of my cheek to stop myself from crying. I’m thinking in rock clichés. I’m thinking of ‘Jugaband Blues’, when Syd sings “And I’m much obliged to you for making it clear/That I’m not here”.

Though not at all like the ‘black holes in the sky’, described by Roger Waters, when Daniel looks right at you, there is a never a sense that he’s returning your gaze. It’s more like he’s always staring slightly off into his own dimension, a Sergeant Pepper’s collage involving The Beatles, Frankenstein, Marilyn Monroe, his first girlfriend and muse Laurie Allen, Captain America, the Silver Surfer and all the cultural flotsam that informs his drawings and music.

He frequently defines himself as a ghost or dead person. A 1994 tribute album – featuring cover versions of his songs by Tom Waits, Beck and Eels, amongst others – was released under the title The Late, Great Daniel Johnston. He cites Casper The Friendly Ghost as a huge personal icon. His song of the same name would play on the soundtrack of Larry Clark’s Kids.

“You’ve probably noticed I’m no ghost,” Daniels laughs. “It’s just that I identify with Casper so much. It’s simple. He always had a good attitude and he was always helping people. That’s the way I feel I should be. Though I sometimes ruin things for people instead. I’ve been told that my music bugs some people. I say crazy things that upset them.”

He shrugs. “But I feel like I’m a good person who just wants to entertain.”

Predictably, he is often classed with such otherworldly talents as Brian Wilson and Sly Stone, but even when his illness has peaked, he is not as reclusive as either of those lost souls. In 1990, in the throes of severe manic episode, Daniel broadcast a one-hour special down the telephone from a mental hospital in West Virginia. This now legendary show, multi-tracked on a double cassette deck, and often spoken of as a Gen-X War Of The Worlds, featured elaborate self-deprecating sketches, a rendition of his track ‘Speeding Motorcycle’ with Yo La Tengo and calls from the listening public.

One caller was Jeff Feuerzeig, director of 1993’s Half Japanese – The Band That Would Be King. The filmmaker promptly determined to make a documentary of the underground hero’s life. 15 years later, and Feuerzeig’s tremendous award-winning film The Devil And Daniel Johnston has successfully sifted through the madcap circumstances of Daniel’s life to provide a riveting, sometimes terrifying portrait of creativity and madness.

Daniel Johnston was born in Sacramento, California, the youngest of five children. His family, described in the film as a ‘Christian fundamentalist Glass family’, soon relocated to West Virginia. A remarkable precocious talent, by his teens, Daniel was operating out of his parents’ basement, making short films, cartoons and DIY albums on cassette. Initially unable to duplicate tapes, he would perform entire albums from scratch for each fresh recording. Already mindful of his future legacy, his compulsion to self-document rarely saw him without a tape recorder or super-8 camera to hand.

“My best friend at school told me everyday that someday I’d be famous. He told me that one hundred million times. I thought, ‘Yeah, yeah’. But growing up, my dad was an architect and I saw how hard he had to work. I saw the people at our church working in the local steel mill and I used to wonder what I was going to do. So lazily I decided I needed to be an artist.”

His illness began to manifest itself while he attended junior high school. He drew armies of all-seeing eyeballs and ducks to fend off Satan, the primary antagonist in Feuerzeig’s documentary. After art school, where he met Laurie Allen, his lifelong unrequited love, Daniel ran away with the circus, arriving in Austin, Texas just as the garage punk scene was exploding. His early albums Hi, How Are You and Yip Jump Music showcased a remarkable gift for pop melody and heartfelt surrealist lyric, despite, or perhaps enhanced by, unsophisticated chord-organ production and his strange reedy voice. He quickly became a cult figure, appearing on an MTV special, but his inner demons were on the march.

By 1988, with the patronage of Sonic Youth, he travelled to New York as the darling of the alternative scene, but his increasingly paranoid delusions caused him to see the devil in every stranger. During one psychotic episode he was arrested for drawing hundreds of Christian fish symbols on the stairwell of the Statue of Liberty. That same year he rushed up to a second story apartment determined to save the soul of the elderly woman who lived there. Terrified by this attempted ‘exorcism’, she jumped from her window, breaking both her ankles.

In the film’s most poignant scene, his father Bill breaks down recalling a particularly horrific incident from 1990. Bill, a pilot during WW2, was flying home from a concert in Austin in his two-seater plane, when Daniel lunged for the keys in the ignition and threw them out the window. Miraculously, Bill managed to land the plane in trees and they survived with minor injuries. Emerging from the forest, they came to a rural church with a billboard that read – “God promises a safe landing, but not a calm voyage.” Photos from that day depict Daniel as gleeful, convinced he had saved his father from Satan.

“That was me, alright”, he says sadly. “I can remember it all. I was reading a Casper comic and he and Spooky had a parachute and I just got confused in my mind, you know? You won’t make the article all about the plane crash, will you?”

No, I won’t.

“It’s just… well, I love the movie. I think it’s really funny. But that bit reminded me of those celebrity shows where they really go for you. It was tragic. We got lucky. My dad is a great pilot. He’s Captain America. So the wings got caught in the trees. But that was the lowest point for me.

To be fair, Feuerzeig shows remarkable sensitivity in The Devil And Daniel Johnston. Tellingly, the director decided not to interview Daniel directly, opting instead to piece together recent footage, old home movies, audio recordings and interviews with family, friends and such notables as Daniel’s selfless former manager, Jeff Tartakov, Austin Chronicle editor Louis Black and Butthole Surfer Gibbey Haines, who pontificates, hilariously, from a dentist’s chair while receiving four fillings.

As a result, Daniel haunts the film more than he inhabits it, though the heavy reliance on his multi-media scrapbook recalls the intensely personal auto-therapy of Tarnation or Capturing The Friedmans. One is also tempted to cite Terry Zwigoff’s remarkable Crumb, but by contrast, Fueurzeig seems determined, rightly, to pull Daniel out from under the unsatisfactory umbrella label ‘outsider artist’.

Certainly, Daniel is no Henry Darger-like savant. In person, he’s easily distracted but perfectly lucid, capable of discussing favourite films and artists like any other rock-star.

“Dylan was an important influence,” he tells me. “A lot of my record Songs Of Pain was inspired by listening to Dylan. Desire and Shot Of Love especially. Then Slow Train Coming, with that religious conversion, really blew my mind. But The Beatles are tops for me. I study everything they do. I have around 2000 Beatles bootlegs. I had more but I had to sell them a couple of years ago because I needed the money. Everything they did is supreme art.”

Though oddly angled, Daniel’s astonishing output as a musician, cartoonist and artist has long attracted celebrity champions and die-hard enthusiasts. He was Kurt Cobain’s favourite singer-songwriter. During his final years, the Nirvana frontman was rarely spotted without his Hi, How Are You t-shirt. Simpsons creator Matt Groening – who appears in the film – has also been a Daniel evangelist since his rock critic days.

Over the past two decades, however, Daniel’s flirtations with mainstream success have been thwarted by instability. In the great post-grunge record company stampede, he refused to sign with Elektra because he believed would-be labelmates Metallica were coming to beat him. Spielberg’s people have also called, to no avail, and a brief deal with Atlantic records, producing the 1992 album Fun, was scuppered by worsening health and poor sales.

Happily, things are finally coming together for this once tortured artist. His artwork, recently selected for an exhibition at London’s Aquarium Gallery and the Whitney biennial in New York, is currently inspiring bidding wars all over the planet. He’s equally excited about his brilliantly raw new album, Lost And Found, released here on May 1 by Sketchbook Records.

“Yeah,” he smiles. “Everything is really happening for me. I’ve just bought a new house. I’ve built it right next door to my parents because I couldn’t do without them. My dad is my manager now. I’ve had some other managers but as soon as my dad took over, I was rich and travelling all over the world. Now, what I really, really want to do is get enough money to buy my dad a B-51. That’s what he flew when he was with the Tigers during the war. I’d like to get him a restored model. He’ll flip out when I do.”

Finally poised for world domination, it is a relief to see him in reasonable health. After a time with him, he appears less like a child and more like an outsized teen, almost endearingly arrested at the point when his illness took hold.

“I’ve finally found the right medication,” he chirps. “I’ve been a manic-depressive since junior high school. When it first happened I couldn’t figure out what was wrong with me. I was dead. Nothing made sense anymore. Then I read about manic-depression in a book and realised what was happening. But after all these years I’ve got the right doctors and the right medication. I’ve been well for a few years now. My moods are finally under control.”

Just before the nice man from the film company tracks us down, I ask Daniel about ‘Lonely Song’ on Lost And Found, which contains the poignant lines – “If you only knew/What I went through/To bring you a lonely song”.

Is the trade off worth it, I wonder, his condition for his gift? Is he getting a better deal than Robert Johnson?

“Oh yeah”, he exclaims. “I make a good living. I’m doing exactly what I want to do. I travel all over the world. I’m doing good. I can’t think how things could be better.”

Let’s all hope it stays that way for Daniel, The Friendly Ghost.