LOS ANGELES — The billboard is huge and Jack Black, rolling down Ventura Boulevard in a red Tesla, can’t resist taking note.

“Now you can write about how famous Jack Black is,” he says with an exaggerated pause, making it clear he finds it mildly hilarious that his mug hovers over Encino. “I wasn’t trying to drive by that. It just happened naturally.”

The ad is for “The House With a Clock in Its Walls,” a $42 million children’s fantasy film he’s made with actress Cate Blanchett and director Eli Roth. It ruled the box office on its opening weekend. If you press Black, he’ll kick out the kinds of quotes (“I was really excited to be a warlock; this is a character I’ve always wanted to play”) that pass for story meat at a Hollywood presser. But “Clock,” you should know, is not the Jack Black movie the actor is most focused on this fall.

That would be “Post-Apocalypto,” an animated musical that Black says cost him $58.93 in pads and pens. The stop-motion saga stars his mock rock duo, Tenacious D, as they deal with the aftermath of a nuclear catastrophe. The comedy is joyously adolescent, packed with over-the-top sex and senseless violence as well as an unexpected, political twist. How much does Black expect “Post-Apocalypto” to earn? Try zero.

Then again, “Post-Apocalypto” is not a business move. The movie, which Black and “D” partner Kyle Gass worked on for two years and both paid for, is being posted on YouTube, one chapter at a time. It’s a creative act and a source of artistic pride, a DIY statement from an actor whose success rarely allows him to step outside the machine.

“There wasn’t a committee on this thing,” says John Spiker, Tenacious D’s bassist and sometime producer. “The committee was Jack and Kyle. That’s why Jack is so amped up about it. I think he feels like, for better or worse, it’s probably the purest artistic expressions he’s been able to put out to the world. It’s a thousand times smaller than a ‘Clock in its Walls,’ but it feels a thousand times more his child.”

“Post-Apocalypto’s” first chapter arrived Sept. 28 and will unfold though the fall until the release, on Nov. 2, of the fourth Tenacious D album, the movie’s soundtrack. Black and Gass will be on tour, as well. Ask Black how he can be so focused on “Post-Apocalypto” at the same time as “Clock” and he’ll shrug.

“We’re just the little engine that could,” he says. “We need to fan those flames. The ‘House With a Clock?’ It’s a huge, mega ocean liner. They don’t need me at all. But Tenacious D is this little passion project that could easily be lost in the shuffle of the billions of things that are out there on the Internet.”

The story of “Post-Apocalypto” is also the story of Black, who, at 49, remains as bankable as ever. His films have grossed more than $2 billion since he played a snotty record store clerk in 2000’s “High Fidelity.” Still, he often talks of retiring from movies, a pledge he will admit is only half-serious. He is all in when it comes to the new Tenacious D project.

For the uninitiated, the duo rocks in all forms, moving effortlessly from folk to prog to thrashy metal. And if the lyrics are often comic, the production itself is as thick as any radio hit. They’ve worked with Dave Grohl, Dust Brother John King, and also featured guests as diverse as Phish keyboardist Page McConnell and multi-instrumentalist Jon Brion. Tenacious D has also had considerable commercial success. Two of their three albums cracked the Billboard Top-10.

But “Post-Apocalypto” is something different, planting Black and Gass — or their alter egos Jables and Rage Kage — into a road ad­ven­ture across a landscape destroyed by an atomic bomb. They encounter monsters, procreating cave women, a two-headed dog named Hope and assorted real-world characters along the way, all of it testing their resolve to remain the greatest rock band that ever lived.

Black could easily have farmed out the work in “Post-Apocalypto,” hiring illustrators to punch out his brainstorms. Instead, he did all the sketches, more than 3,000 in total, with a ballpoint pen. Spiker scanned them into his Mac so that they could be colorized.

“It took forever, but I really did enjoy the process,” Black says. “In many ways, I preferred that to going to set and working on a movie. My hope is that it’ll play like ‘Beavis and Butt-Head.’ Those are bad drawings, but they’re hilarious. I secretly hope this is successful enough that I’m asked to do more drawing.”

If Bobby McFerrin and Dio had a love child

“Post-Apocalypto” is the perfect Venn diagram for Black, bringing together his love of drawing, music and theatrics.

He started drawing early. Before preschool.

“And he was prolific,” says Tom Black, his father. “We have hundreds, if not thousands, of his drawings.”

Music also became important. His older brother, Howard, a recording engineer, took him to see geek punksters Devo during the band’s “Freedom of Choice” tour. The theatrics, the pre-set film, all would be deeply influential. Black also became obsessed with Bobby McFerrin, punching out his own a cappella harmonies on a four-track.

“Lots of sounds and things coming out of his bedroom door,” says Linda, his stepmother. “I was like, ‘Tom, there’s something wrong with him.’ I’m from Iowa. I never raised a kid who had all that energy and creativity.”

Black did struggle. He got into trouble at school, got into cocaine and found himself the target of a bigger kid. His parents placed him at Poseidon, a school for troubled kids in Los Angeles. It was there that he met one of his mentors, theater teacher Deb Devine.

“He came into my class with his arms folded, wouldn’t cross the threshold and he slowly worked his way in over time,” Devine says. “He’ll tell you I saved his life, but I believe half of that was his own incredible talent, his own personal talent that needed to emerge. Can you imagine having Jack Black in your theater class?”

By his junior year, Black left Poseidon and headed to Crossroads, an elite, private high school whose alumni include Gwyneth Paltrow and Zooey Deschanel. He starred in “Pippin” and a student film by classmate Brett Morgen, later an accomplished documentarian.

He tried UCLA but dropped out his sophomore year, joining Tim Robbins’s Actors Gang. That’s where Black met Gass, who was eight years older and into Neil Young and the Eagles. Gass taught Black how to play guitar. Black preached the power of metal.

Early on, the band wrote “Tribute,” a self-referential origin epic that opens with Gass plucking in a minor key and Black promising “this is the greatest and best song in the world” before revealing that it actually isn’t. Everything great about Tenacious D — soaring harmonies, thrashing guitar, goofy spoken word — can be found in that song.

“I think the one binding thing was the bravado,” Gass says. “It was the mask, it was the armor that we needed because we’re both tourists, too. We’re not really band guys. We’re shlumpy guys. We don’t look like rock stars. And I only really like to play the acoustic guitar.”

Tenacious D’s first break came on “Mr. Show With Bob and David,” the comedy variety program Bob Odenkirk and David Cross started in 1995. That led to a series of episodes on HBO, which began airing in 1997, and a gig in 1998 at the Viper Room. Foo Fighters leader Dave Grohl was there that night. He couldn’t believe what he heard. He would effectively become the D’s studio drummer.

“If Bobby McFerrin and Ronnie James Dio had a love child, it would be Jack Black,” Grohl says. “He’s totally capable of singing like a professional opera singer. He can stretch, become someone else on screen, but the first time I met him was at the Viper Room. I spent the next 45 minutes rolling on the floor laughing but also blown away that they were so talented.”

Crickets, then control

The first time Grohl heard about “Post-Apocalypto,” he was confused.

“I just assumed they were going straight to Broadway with the thing,” he says. “I didn’t realize it was going to be notebook sketches animated to Tenacious D music.”

Black cooked up the idea back in 2016, even before filming “Jumanji: Welcome to the Jungle.” That film came out in 2017 and earned nearly $1 billion. Tenacious D continued to work on what would become their first project since 2012’s “Rize of the Fenix.” Early on, Black pitched the concept of “Post-Apocalypto” to Netflix, Amazon and HBO.

“Look at this gem,” he says. “They’d be idiots not to buy this thing. Let the bidding war begin. And crickets. No one was interested.”

He is joking about it now, but Tanya Haden, his wife, could tell the rejection hurt. She gave him advice.

“I said, you can just do it yourself and put it out on YouTube,” she remembers. “Just put it anywhere you can. Just do it. Because that’s what he told me. Do it to have fun. Don’t worry about who is going to like it.”

Strangely enough, it was Gass, the less famous partner, who worried about image.

He thought Black’s penchant for drawing graphic sex scenes might be too much. He also felt funny about using the D to take a political stand. There is a clear, anti-Trump thread in the film.

“Here’s what I was worried about,” Gass says. “We were going to be labeled smut peddlers and I was like: ‘Jack, are you sure you want to jeopardize the brand. Because the kids love you and the parents love you.’ ”

Black finds the whole thing funny. His brand?

“This could be the reason I don’t get to be in ‘Jumanji 2’?” Black says. “Part of me’s like, bring it. If that’s really going to happen. I almost have that defensive posture. I want to get in the fight. I want to get blacklisted. Because then you’re on this list with these other righteous people.”

A monster named Cracka-lacka-ding-dong

“The vagina monster was a huge breakthrough for me,” Black says.

It is Saturday night and he’s sitting at a diner near his house, enjoying a burger with Swiss cheese and talking about the monster, named Cracka-lacka-ding-dong, that the D must fight in an early scene. Black is asked about his professional future and admits he’s not totally sure. Sometimes, he just wishes his Google calendar would be blank.

The movies are fine, though he doesn’t sound desperate to get back on a set. Sure, doing something with David Lynch or Jim Jarmusch would be cool. But as he approaches 50, Black brings up Jack Nicholson.

“He’s not going to do another ‘Cuckoo’s Nest,’ he’s going to do another ‘About Schmidt,’ ” Black says. “It’s time for my ‘About Schmidt.’ ”

Then he gets back to the monster, which is actually more of a vagina, penis combo beast.

“It’s like, what is this monster going to look like, going to sound like?” he says. “And every chapter, I had to top the previous vagina monster. It really got hard in Episode 5. What’s he going to look like, what’s he going to sound like?”

He mentions “The Exorcist.”

“When he’s talking to the demon or he’s walking down the street and he’s having this dream of a demon. There’s this moment where it’s black space and just a flash of a demon face. It’s not a skeleton, but it’s terrifying. One frame. Maybe it was two or three frames. And I went back and found that demon face and I just copied it. I just stole it.”

He finds that funny, just as he planted a robot in “Post-Apocalypto,” who identifies himself as a “terminator sent from the future.”

“That’s the truth. Why am I so proud of this thing? It’s really just a hodgepodge of stolen things. But,” and here he smiles and offers at least a glimpse of those devilish eyes, “it’s my hodgepodge.”