Don’t believe everything you hear about Sean Penn, the Oscar-winning actor, director, human rights activist, and cofounder of JP/Haitian Relief Organization. There is no evidence of Penn’s media reputation as a “bad boy” when he sits in his Los Angeles home for an interview about his first book, the novel Bob Honey Who Just Do Stuff (Atria, March). The den, which also holds Penn’s impressive book collection, is filled with framed photos of his family and friends. He greets me warmly and gives generously of his thoughts about writing, the book business, and what motivated him to write fiction in the first place.

“I had directed a movie, working with people who were very talented, and, for the first time on a set, I was miserable,” Penn says. “And didn’t know why.”

The leading cast was a bit younger than Penn, he says, and perhaps lacked the maturity that comes with his experience. “I found myself feeling very much like another generation. While I had their talent available from the time they checked in until they checked out, they had business calls to make; they’re sending emails, texts, whatever it was they had to do. Despite their talent, they seemed more interested in selling films than making them. You’re massaging egos everywhere. Richard Harris said, ‘I’m the last of the generation of actors who doesn’t go to bed at 10 p.m. with face cream on.’ I felt very much that way on this movie, which became an extremely terrible experience. It made people that I knew well just hate me.”

Penn went into a depression. When it passed, he knew it was time to do something where he wasn’t disappointing anyone and began to write Bob Honey. “I had a blast,” he says. “I giggled a lot. I went to a place in a rhythm, and felt I was free to find what words turned me on. I’m a big melody guy, and I know the clues. Some people you don’t understand, but if you listen to their melody, you totally understand what they’re saying, who they are, where they’ve been, and what they do for a living. That was a driver to some of the book.”

Penn is the first to admit he has entered an industry that he knows almost nothing about. “I’ve had my success in movie stuff and everything, and I’m a known person, and in some ways that makes it harder,” he says. “I wrote a novel at 56 years old, thinking, what am I going to do, go to a publisher and say, ‘Hi, I’ve written a novel’?” So he took a different route and first signed a deal with Audible.

Because Bob Honey shows contempt for the politically corrupt state of America today, Penn wanted to get the book out in some form before the 2016 elections. “So I recorded it in a rush, and then I went to the Los Angeles County Museum of Art and read it in front of a live audience. That wasn’t recorded, and I didn’t like the way I sounded on the Audible version, so it no longer exists today.”

Six months later, Penn rewrote and expanded Bob Honey, and his agent, Mollie Glick at CAA, sold it to Peter Borland at Atria. “He was fantastic,” Penn says. “He accepted all my changes, and every idea he had was so in tune with mine. I hadn’t had an experience like that in the movie business.”

Penn was surprised that they didn’t spend more time working together. “I thought it was going to be a Maxwell Perkins kind of thing, where we’d sit in a smoky room together for a year,” he says, laughing. “But no. Once every two weeks Peter would send me a note, what do you think of this or that, and they were always dead-on. There will be people who loathe the book, and people who will love it, and it’ll do what it’ll do.”

Penn leans forward on the sofa. “This whole world is so new and different to me,” he says. “I call it Spain, because people sure do take siestas in the book business!” He grins, adding, “What do you mean, you can’t get this read by then? What do you mean, you left the office at two? In the movie business it’s grind, grind, grind.” He punches his palm for emphasis. A shaggy yellow puppy bounds into the room, just back from a trip to the vet. Penn sips his coffee and waves goodbye to his son, Hopper, who ambles out of the house.

The eponymous hero of Bob Honey is an unhappy middle-aged man who has trouble connecting with other people. He lives in Northern California but travels to the Middle East and South America for his job.

“Bob is an undercover assassin for a covert government organization,” says Borland, Penn’s editor. “He’s tasked with eliminating the elderly—they don’t add enough value to the economy—and is at odds with the world around him—politically corrupt, environmentally damaged, culturally unhinged—that is less tempting every day.”

Borland compares the book to Allen Ginsberg’s poem Howl, because Bob rails against the things in our culture that make people inhuman. Toward the end of the book, he drives cross-country to save his girlfriend and is shot during a violent confrontation in Florida. He survives and afterward has a life-changing epiphany. Borland adds, “Bob re-enters the world without regrets or fear, becoming the man he was meant to be.”

When Penn is asked to describe his book, which he wanted to write under a pseudonym (“I like pseudonyms—I used to check into hotels under the name Molly Ringwald”), he says, “It’s about a person in the modern age making the unusual demand to be human.” The story closes with a scathing rant by Bob about a political figure, Mr. Landlord, who sounds remarkably like—guess who?

Penn acknowledges the similarities between writing and acting. “What I found is that, when I write, it’s like performing alone,” he says. “I’m hearing and seeing it, and I know what it is that’s going to tell the story. I’ll recognize different approaches to the same thing that will also tell the story. So in terms of giving an actor freedom, I’ll recognize how to lay out my ideas and how to talk about them with that actor. But in writing, it always comes back to the melody I hear in a voice. The only difference between characters boils down to personality.”

Penn skillfully weaves a variety of colorful characters into Bob Honey, including a nosy neighbor who complains to the town’s police department about Bob’s “unusual” behavior, a menacing operative named Spurley Cultier, and “Fischel the Jew,” whom Bob rescues from political captivity in Bolivia.

So how would Penn pitch Bob Honey to a book buyer? “When Martin Scorsese did the documentary about the Band, The Last Waltz, the first words you see on screen are, ‘Play this movie loud,’ ” Penn says. “Bob Honey is that kind of book. And I would say this is a book you have to read holding it in your hands. Don’t sell it to anyone who’s going to read it any other way.”