Sean Penn. Picture: Photo by Willy Sanjuan/Invision/AP

Stephen Colbert has a habit of supplying his "Late Show" interviewees with booze. In February, a barefoot Jennifer Lawrence drank a whole lot of rum while chatting about "Red Sparrow." Earlier this month, notorious strawberry hater Tom Brady chugged a beer in one gulp.

Sean Penn, on the other hand, brought his own vice to the show on Monday night. After telling Colbert that the host had "inherited a little bit of the Ambien I had to take to get to sleep after a red-eye last night," Penn pulled a pack of cigarettes out of his jacket pocket and lit up right there.

"So, in other words, you're still a little bit on the Ambien train right now?" Colbert asked. A smiling Penn responded, "A little bit."

Penn appeared on the show to promote his satirical debut novel, "Bob Honey Who Just Do Stuff," and said that he doesn't want to act anymore. Acting is all about collaboration, he said, and "I increasingly don't play well with others." In writing the novel, Penn got to work in solitude.

"I was never disappointed with me," he said.

This isn't the first we've heard of "Bob Honey." Back when Penn still collaborated with others - during the 2016 election season, specifically - he, Frances McDormand and a few others recorded a free audiobook version of the story. It credited someone named Pappy Pariah as the author, whom Penn, in a past appearance on the "Late Show," allegedly claimed he met at a writers' conference 40 years ago. Colbert remained skeptical, though, and on Monday night got Penn to admit that he had lied.

"When I first thought about this book, I started to hear a rhythm of speech, a storyteller about Bob Honey," Penn said. "So I gave him a name, Pappy Pariah, which became a character in the novel, and I decided to let it live as his for a bit. And now, I'm here, in an evolution of that process, sharing the development of the earlier truth I told you."

Did that clear things up? Probably not. But an amused Colbert, calling back to the Ambien, told Penn, "I'm on board. I'm on that train with you."

After calling his cigarette habit "job security for oncologists" and listening to Colbert ruminate on how pretty the word "pariah" sounds, Penn ended the interview on a serious note. A known activist, he praised the high schoolers from Parkland, Florida, who organized last weekend's March for Our Lives.

"Within days of that, not only are they stating their case with incredible sobriety and articulate words, but in such an inclusionary way," he said. "You feel like there are reasonable people who have been on the other side of this conversation who will listen to these kids."