While you learn a lot about Conan O'Brien when you spend time on his set, we also wanted to talk to the people who have known O'Brien over the course of his 23 years on late-night television. Many of them are celebrities.

We reached out to quite a few of O'Brien's frequent guests and collaborators over the years, from his former bosses to interns. Here are some of the things we learned.

Some people think his sidekick, Andy Richter, is too mean to him.

Richter thinks that's ridiculous. "I'm not at all intimidated by him, because I know he's the same goofball that I met in 1993. And yeah, he's in charge of this whole enterprise, but he's still just a guy," he said. "And if I'm going to be on TV with a guy, I need to maintain this relationship with him of 'he's a guy.' He's not all-caps 'CONAN,' he's Conan, he's that guy I've been on TV with for 20 years, on and off." Richter added, "I see people sometimes and they're like, 'Oh man, the way you tease him or take shots at him.' I think people could take more shots at him. People could tease him a lot more and have a lot more fun with him, and I think in some ways he would thrive on that."

Will Ferrell will always be grateful that O'Brien let him be weird. 

Back in the "Late Night" days, Ferrell was on "Saturday Night Live" and would just walk down a couple flights of stairs at 30 Rockefeller Plaza and stop by the show. He always brought his most bizarre ideas, such as dressing up as Mr. Clean. "Some of the other 11:30 talk shows were a little more protective about, 'Oh is that too weird, we don't know if we can do it.' But Conan was so supportive of me and the out-of-the-box comedy ideas that I would have," Ferrell said. "The fact that they let me appear as Robert Goulet in character for an entire segment is just like unheard of. They didn't even question it. They were like, 'Great, yep, love it.' And I kept referring to him as Johnny Carson the whole time. Like, that just doesn't happen."

"The Simpsons" creator Matt Groening didn't have any hard feelings that O'Brien left to go to "Late Night."

… and not just because O'Brien would visit his former workplace and take the staff out for pizza. "His humor is so charming and self-deprecating and he is just a good guy," Groening said. "In fact, I tell him, in his honor, his old office is now a supply closet. No one can replace Conan's office, so it's now supplies."

"Black Drinking Day" came out of a casual conversation, according to Deon Cole. 

Some of O'Brien's memorable bits come out of random chats between writers. Cole recalls that happened with the idea behind "Black Drinking Day," Cole's first bit he did on "The Tonight Show" about how African Americans needed a day designated to drink, too. (Think Cinco de Mayo.) "I didn't even think of it as if it was on-air worthy, just a conversation that led to it being a thing," Cole said. If you're wondering, Black Drinking Day is Oct. 7.

It took Greg Daniels awhile to get used to the idea of his college pal as a late-night TV star. 

Daniels (creator of "The Office" and "King of the Hill") and O'Brien met during a freshman-writing seminar at Harvard and eventually moved out to Los Angeles together to take a job as writers on HBO's "Not Necessarily the News." When O'Brien landed his late-night job as a complete unknown, Daniels thought his friend handled sudden fame well, though it took some adjustment on his part. "It was still weird when it became difficult to walk down the street without people yelling at him," Daniels said. "Or when my first child was being born and the TV in the delivery room was stuck on Conan. No matter how much we begged the nurses to protect the newborn from Ed Koch jokes there was nothing they could do."

Jack McBrayer doesn't mind when O'Brien makes fun of him — really. 

McBrayer's first 30 Rock memories (before, you know, "30 Rock") was going to "Late Night" to play various characters. He was broke back then, so "Late Night" helped him pay his bills: If you had zero lines in a sketch you got paid $300; if you had two, you got $550; if you had more than five lines, you got $1,000. Even when McBrayer comes back as a guest, O'Brien loves making fun of him. "My gosh, he enjoys attacking me, that's for damn sure, and I enjoy it just as much," McBrayer said. "It's so funny and weird to me, he really truly gets joy out of watching me squirm. I know he's joking and I know he's not doing it maliciously… [but sometimes] I don't think people get it."

O'Brien almost made John Krasinski cry on-air. 

Krasinski was a script intern on "Late Night" in college; years later, he and his wife, Emily Blunt, are friends with O'Brien and get together for dinner. But Krasinski had a very emotional "full-circle" experience when he got a lead role on "The Office" and then went back to "Late Night" — as a guest. "To this day, it's the most surreal moment of my career," Krasinski said. "I legitimately blacked out… I walked to the couch, and when I came to, Conan was shaking my hand," he said. O'Brien told him, "Dude, sit down, it's all going to be fine." During the interview, O'Brien told the crowd Krasinski was a former intern and that he was very proud of him. "My eyes flooded with tears," Krasinski recalled.

Marc Maron says O'Brien's "nervous intensity" is inspiring. 

O'Brien, who has appeared on Maron's podcast, was one of the first people who put Maron on-air as a stand-up. Maron still enjoys going back to visit the show today. "He doesn't strike you as an inherently comfortable guy, and that in and of itself is unique," Maron said. "That nervous intensity mixed with that incredible intelligence and vast memory… he's a very bright guy." But it helps when you're bantering back and forth in an interview: "If Conan's there and he's egging you on with his nervous intensity, you want to meet him there, you kind of want to show off more."