“I first met Karen Black in 1970 in New Mexico, on a film called The Gunfight, with Johnny Cash and Kirk Douglas,” says actor Keith Carradine, 71, calling from his Dodge Charger on a warm California afternoon. “I was 20, not old enough to drink, too young to vote, a young man with typical interests and I was stunned. Here was this beautiful, beautiful woman. I was intimidated. I was shy. I shut up and I paid attention.”

She would soon be directed by Robert Altman, Alfred Hitchcock, Jack Nicholson, John Schlesinger and more, and earn three Golden Globe nominations, but at 30 years old, with a decade of theatre and TV roles behind her, Black was only just establishing a name for herself in New Hollywood cinema in 1970. The Illinois-born actor had helped Francis Ford Coppola with the making of his first film, You’re a Big Boy Now; and played the flame-haired hooker in Easy Rider, tripping on acid in a New Orleans cemetery with Peter Fonda, Dennis Hopper and Toni Basil. Most importantly, she had just starred opposite Nicholson as singing waitress Rayette in Bob Rafelson’s meandering 1970 picaresque Five Easy Pieces – a role that would earn her first Golden Globe and an Academy Award nomination for best supporting actress.

“That must have been the film Robert Altman saw that convinced him to cast Karen in Nashville,” says Carradine of Altman’s 1975 music industry satire in which the pair both star as country singers. “She was just frickin’ brilliant, you know? She’s acting, improvising and when she sings, she’s doing it on a level that is legitimate, and would have been acceptable at the Grand Ole Opry. First time I heard her sing I said, ‘Oh yeah. She’s got it.’ I mean, I got a record deal with Asylum Records after Nashville but I’m only hearing now, from you, that Karen went down the same path.”

I’d been telling Carradine about Dreaming of You, a collection of song demos that Black recorded with producers Elliot Mazer and Bones Howe in 1971 and 1976, now unearthed by her husband Stephen Eckelberry and California singer-songwriter Cass McCombs.

“I’d become aware of Karen Black in high school,” explains McCombs, 43. “I worked at a video store and I was obsessed with her movies. Then I met her through some film-maker friends in 2009 and asked her to sing on one of my songs, Dreams-Come-True-Girl.”

The pair stayed in touch and started collaborating on an album. “When she was singing she was really self-deprecating but in a charming way,” says McCombs. “She created this casual space where you could just let go of all your egos and assumptions. It was fun. The vision was to make a complete studio album. She told me that she’d been trying since the early 70s.”

“She’d done some demos with Elliot Mazer in Nashville in 1971,” explains Eckelberry, “and the next thing you know she’s filming in New York opposite Richard Benjamin in Portnoy’s Complaint, she can’t turn that down. She’d do the [recording] sessions then forget about them. I mean, she loved singing but she was an actress first.”

But if the acting came naturally, the networking did not. “She’d call up people in the industry just to say hi,” says Eckelberry, “Which is weird in Hollywood, where everybody wants something. It worked against her in some ways, because she’d just follow her interests. I remember the after-party for [Jack Nicholson’s 1990 Chinatown sequel] The Two Jakes. A chance to network, right? Karen literally spent the night having an animated conversation with the doorman. She wasn’t built for business.”

As a result, the 90s and 00s saw Black working primarily in little-seen independent projects, and low-budget thrillers and horror movies such as Steve Balderson’s Firecracker (2005, starring opposite Faith No More singer Mike Patton) and Rob Zombie’s House of 1,000 Corpses (2003).

“She was never generic,” says Rob Zombie. “There was always something odd, free and uninhibited about her. When I cast her as Mother Firefly in House of 1,000 Corpses, she was someone outside the normal thought process. She was kooky, eccentric; you’d be talking to her off camera and thinking, what the fuck is she talking about? And then boom, action! And she would nail her lines perfectly every time. She’s just terrifying in that film: overly friendly and completely insane at the same time; totally inviting and totally demented.”

Zombie and his wife, the actor Sheri Moon Zombie, both stayed friends with Black after the film finished. “We’d go for dinner or go see a new movie she’d made,” he says. “I mean, the industry had changed a lot from the 70s and 80s and you saw that. She was from a time when people cast Dustin Hoffman or Michael J Pollard as leading men. Not Vin Diesel. But if Karen wasn’t getting the same kinds of roles it wasn’t for lack of talent.”

“What never went away was that she loved being on camera,” says Eckelberry. “I filmed her for the last three years of her life as much as I could. I filmed her while she was dying of cancer. Somebody asked me, ‘Isn’t that kind of cruel?’ I said no, because to Karen when you’re making a movie things aren’t so bad.”

When Black died in 2013, McCombs dedicated his album Big Wheel and Others to her and set about trying to put together a posthumous collection of songs.

“The original plan had been for [late American record producer] Hal Willner to work with Cass,” says Eckelberry, “and bring together all the songs Karen had sung in movies throughout the years, but nothing came of that. Then I’m moving house in 2015 and I’m confronted by boxes in the garage. Karen was a pack-rat and she boxed up everything, and there were boxes and boxes of tapes full of songs.”

“The tapes were completely mouldy and degenerated,” says McCombs. “Not everything was salvageable.” Luckily, the songs from her two 70s album sessions were saved. “My original idea was to adopt the Beatles’ Free As a Bird approach,” explains McCombs, “you know, playing over the top of these old songs. But listening back, living with them, meditating on them, I grew to love them the way they were, just so intimate and real. I couldn’t improve on them.”

If you’ve seen Karen Black in any of those early-70s films such as Five Easy Pieces, Nashville, Born to Win or Cisco Pike, you’ll have heard her songs, self-penned for each film and delivered in a hypnotic, beguiling voice that moves with uncanny ease from warm and intimate Dolly Parton soprano to a haunting falsetto.

“Karen sang all the time,” explains her friend, the choreographer Toni Basil, who first met Black when the pair were filming Easy Rider in New Orleans in early 1968. “So directors would hear her on set and suddenly they’d want to add a scene to include her singing. I assumed she was a professional singer. I didn’t feel as though it was something that she was hesitant about. She approached songs from an actor’s point of view. She’d perform each song as a character. She’d inhabit these songs.”

Inhabit is a good word. Whether on her self-penned tracks or her startling acoustic cover of the Moody Blues’ Question, there is an eerie intimacy to the songs on Dreaming of You. While her movie voice could be gentle, inviting, here it can also suggest something a little more unsettling, an attenuated spirit cry with just a little dash of folk horror at the edges. A good comparison might be Broken English-era Marianne Faithfull, Appalachian folk singer Hedy West, or the early-70s home recordings of German actress Sibylle Baier; spellbound female voices possessed of an uncanny emotional honesty.

“To me, those songs just sound like Karen,” says Eckelberry. “She was like that in real life. She couldn’t help but mean everything. I think Cass saw that in her. He just loved her, and I think that’s what made him want to persevere with this project, and for that I’m so grateful.”

When I put the same question to McCombs he initially says he just wants people to watch more Karen Black movies. “You know,” he avers, “have a Karen Black film festival, check out Five Easy Pieces, and the songs. It can only enlighten us, give a little bit more insight as to who she was.”

Who was she, I ask. “A shapeshifter,” says McCombs. “I mean, there were a lot of different emotions going on. But on a basic level, she was always sensitive to what she was doing within the musical space. She’s was attentive. That’s rare. Even professional musicians don’t always have that quality.”

As for where next, both Combs and Eckelberry say that they haven’t ruled out a second volume, depending on what tapes are unearthed next. “It’s a rights thing,” says Eckelberry. “It’s complicated. We’re still trying to license all the songs she recorded for various movies. Also, we only allowed one cover on the album, which was Question by the Moody Blues, but you need to hear her cover of Paul McCartney’s Junk. Wow, that’s so good.

“But who knows. There was also an empty box we found that just said, ‘Karen Black – Randy Newman’. Maybe Randy has those tapes. Do you know Randy Newman? Could you maybe ask him if he remembers recording with Karen Black?”

• Dreaming of You 1971-1976 is out now on Anthology Recordings.