Men of the '80s & '90s -- Then & Now

See why Colbert eventually told his guest, "Please don't smoke anymore ... those things are bad for you."

Sean Penn lit up social media on Monday night after lighting up a few cigarettes on "The Late Show with Stephen Colbert."

Appearing on the show to discuss his new book, "Bob Honey Who Just Do Stuff," Penn kicked off the interview by explaining why his behavior may be even more random than usual. "I'm doing well. You've inherited a little of the Ambien I had to take to get to sleep after a red-eye last night," he said after taking a seat.

"So in other words, you're still a little bit on the Ambien train right now?" the host asked. Penne's response: "A little bit."

But Colbert wasn't sure how different Penn was actually acting. "How do you tell Sean Penn on Ambien from Sean Penn not on Ambien?" he asked. "You lay back in interviews in a really big way. You're very lowkey."

Pulling out his first cigarette of the night and lighting up, Penn said, "I think it's pretty much the same. There's a lot of times I'm just regular tired." That's when Colbert gave him a challenge: "Jimmy Carter is my guest on Friday, he's 93. lets see if you can bring more energy to this interview than Jimmy Carter's going to bring on Friday. Is that fair?" Penn nodded affirmatively, but -- judging by the rest of his interview -- he may not have understood the question.

The two went on to discuss Penn's decision to step away from acting, which Colbert said may have broken a few hearts out there. "The breaking hearts, plenty of people will be thrilled also," Penn responded. "The greatest thing an actor can bring to the party is to play well with others, the collaboration, and I increasingly don't play well with others, so it becomes less enjoyable. That's why I finally came around to writing a novel, I didn't have collaborators. I was never disappointed with me."

Eventually, Colbert had enough with the smoky set, telling his guest, "Please don't smoke anymore. I don't mind. My parents smoked when I was a child so it gives me happy memories to smell cigarette smoke, but you know we want you to be around for a long time and those things are bad for you." Penn's sarcastic response: "This is job security for oncologists," said as he put out the butt in an ashtray.

He lit up again later in the episode and viewers were quick to point out his behavior on Twitter.

Penn's latest late-night gig comes after he appeared on Marc Maron's podcast earlier in the day, where he opened up about his relationship with ex-wife Robin Wright.

"We don't not get along," he explained. "We have very separate relationships with our kids at this point and it seems to work better that way because they are making their own decisions. As it turned out she and I did not share the same ethical views on parenting, including the continuing parenting of adult children." He added that the two "don’t have a lot of conversation."

As for their kids together, Dylan and Hopper, he said, "They're amazing people. They're both acting and modeling, an industry that I'm not very interested in but they seem to have fun with it."

Sean Penn Brings Kids Hopper & Dylan to "The Last Face" Premiere at Cannes