“Quite frankly, we didn’t know what the hell we were doing,” Mellencamp said.

Thirteen years later they’ve created “Ghost Brothers of Darkland County,” a musical that’s not quite like anything out there — as you might expect from two of America’s most independent artists. Along the way, the author and the singer picked up T Bone Burnett to serve as a general contractor, enlisted stars like Sheryl Crow, Elvis Costello, Kris Kristofferson and Rosanne Cash, and broke several rules in the classic musical theater handbook.

King says he might have given up long ago had Mellencamp not kept rolling things forward. Mellencamp says that’s a bunch of bull. Now that they’re done — “Ghost Brothers” is out this week with a CD box set, mini-documentary and e-book, with a theatrical tour starting in October in Bloomington, Ind. — they say the project strengthened their friendship and left them with a tremendous sense of accomplishment.

“This morning when I went over to my office there was a big stack of the box sets and I looked at that thing and said, ‘We actually have a product here,'” King said. “It’s all been give up to this point. You give of your talent and you give of your time, and then you get something back. It’s here and people are either going to play these tunes or not, buy and download or not, go to see the show when it comes to a town near them or not.”

The musical started with a real-life ghost story. Mellencamp was looking for a cabin on Lake Monroe in his home state of Indiana. As the owners handed over the keys, they casually let it drop that the cabin might be haunted, the spiritual remnants of a terrible tragedy that had happened decades earlier when two brothers quarreled over a girl.

The story came with a stack of ancient pulp magazines that detailed the deaths in grisly detail, complete with photos of a headless body and plenty of purple prose.

Mellencamp scoffed, had the cabin remodeled and took his family to the lake for a long visit. They noticed the “weird vibe” immediately.

“I don’t believe in this stuff,” Mellencamp said, “but stuff would start moving. You’d start smelling cigars. Funny smells would appear and stuff would turn on and turn off. It was kinda creepy, you know?”

Mellencamp unloaded the cabin and eventually relayed the story to his agent sometime before the turn of the century. He’d recently been approached about doing a musical based on his hits, but he wasn’t interested. The agent suggested the ghost story could serve as the basis for that musical, and suggested they contact mutual client King to help write it.

King and Mellencamp had met a few times over the years, and to Mellencamp’s surprise the idea quickly took root. It was just the kind of challenge King likes.

“Once you get to a certain age — I’m in my 60s now — you’ve got to try to keep expanding your field,” King said. “You’ve got to try new things and if you don’t, you tend to get conservative. I always say you dig yourself a rut and then you furnish it. John asked me when we started this if I’d ever done anything like this before. I said, ‘John, yes, I have. I wrote a play for my Boy Scout troop when I was 11 years old. And it was a big hit with my relatives.'”

They traveled to New York together where they took in several musicals on Broadway. And almost nothing appealed to them.

“It was like, how does this work?” Mellencamp said. “What we saw on these musicals, at least to me, was a bunch of s— we didn’t want to do. OK, we don’t want any dancing, that’s the first thing. We just don’t. We don’t want this, we don’t want that. We don’t want to advance the story forward with song, it’s too corny.”

They decided they’d use their songs to color their characters. The spoken-word sections of the musical would drive the story, just as they do in a play.

“I just feasted on that because I’m a big rock music fan and country music fan and alt-rock fan and all that stuff, and I thought, that’s what music does,” King said. “Music speaks to the heart and words speak to the brain, and we can really do something here. We saw eye to eye on a lot of things and one was we didn’t really want this big orchestral, violin-heavy music. We wanted a kind of American soundtrack.”

King roughed out the story about two generations of brothers in fictional Lake Belle Reve, Miss., caught in a tragic tape loop and marked out spaces for songs, sometimes including a little rhyme to give Mellencamp cues. Mellencamp then worked up songs from several perspectives.

That’s where Burnett, the only producer Mellencamp’s ever had, enters the picture about five years ago. Mellencamp played him the songs and, always up for a challenge, Burnett signed on.

“It’s a very interesting group of tunes he’s put together and I do believe some of his best songs,” Burnett said. “Time will be the judge of that but it seems so to me. They’re real powerful tunes, and real stripped down, of-the-earth type tunes. … John and Stephen work a lot of the same turf, the legends of small-town America, that stuff. So I can see the resonance between the two of them pretty clearly.”

Burnett brought in friends and colleagues to hang Spanish moss from tree limbs, add humidity to the air and bring the characters to life. Actors like Matthew McConaughey and Meg Ryan helped with the spoken-word parts. Crow sings from the perspective of the coveted woman at the center of the story. Kristofferson provides a moral compass. And Costello had a great time taking it fire and brimstone.

“I had to be the Devil in that story so I made some very extraordinary sounds,” he said. “I mean I was creaking. I sounded like my teeth were about to fall out. I thought the Devil, that can’t be an ordinary voice. So I sang in harmonic shrieks and whispering and everything.”

The thing that Mellencamp loves about the final product is you can’t pigeonhole it. All the principles will be replaced by other actors and musicians when the musical hits the road around the country later this year. The ultimate goal is to debut “Ghost Brothers” on Broadway, but producers have been leery of that unconventional structure. No matter.

“If it ends up there, great, and if it doesn’t, that’s fine too,” Mellencamp said. “Because the real victory in ‘Ghost Brothers’ is that very rarely do you collaborate with somebody that you walk away and go, ‘You know, I really like that guy. I really had … fun with that guy.'”

AP Music Writer Mesfin Fekadu in New York contributed to this report.