For O’Brien, the open landscape of California may be both inspiring and isolating. In his “Late Night” show in New York, O’Brien was able to define himself as a smart outsider, but “The Tonight Show” is, by its nature, a different game for a different crowd. Traditionally, it has mirrored the mood of the country. “Johnny’s the one I look back to,” O’Brien continued. “The constant is Johnny. He was very sophisticated, but he was also a clown. As a kid, I was fascinated by the fact that while he was clearly the coolest guy in America, he could dress up like an old woman or have a raccoon crawl on his head. He surrendered his dignity, and it only made him cooler. There aren’t a lot of hosts who will put on a skimpy bathing suit and jump into a hot tub with Don Rickles.” O’Brien picked up a guitar. “When I first got ‘Late Night,’ ” he said, “I was whisked to John Cheever Connecticut to a 50th birthday party for Bob Wright, who was then the head of NBC. They said, ‘We want you to get up and be funny.’ And then I realized that Johnny Carson was there, too. I was petrified. Johnny was wearing dark glasses and he was ramrod straight and perfectly coiffed. He was very shy. I wanted to kill myself. And I had to go first. I had prepared a thing — the idea was I didn’t know who Bob Wright was — and it worked. People really laughed. And Johnny nodded. Then he got up, made his toast and blew the roof off the place. Afterward, he came over to me and said, ‘Good luck to you.’ He said, ‘Just be yourself — that’s the only way it can work.’ ” O’Brien paused, “There’s an opportunity to put my stamp on this show. I’ve got an ego, and I want to do my ‘Tonight Show.’ ”

O’Brien was interrupted by Jeff Ross, who wanted him to go downstairs to the stage. When the set was being designed, O’Brien’s main concern was that the space not feel too big. “We want to have a parade of elephants if we need it,” Ross explained as we went through the stage doors, “but we still want the audience to feel like they’re close to Conan.” These decisions matter: for the first two years of Leno’s tenure, Letterman was regularly beating him. Then Leno changed studios, switching from Carson’s old arrangement, which put him at a distance from the audience, to a closer configuration. As a stand-up comedian, Leno played more to the crowd and needed to see faces. It may have been a coincidence or a national change in taste, but when he altered his set, Leno started to win in the ratings. And then, in 1995, Hugh Grant was arrested for soliciting a prostitute and didn’t cancel his “Tonight Show” booking. When an abashed Grant sat down, Leno asked, “What the hell were you thinking?” There was a huge laugh, and ratings soared. From then on, Leno was No. 1.

“Do you think it was the set or Hugh?” I asked Ross.

“Well,” he said, “I couldn’t come up with five names living that will guarantee a rating. News-generated guests will be big, but everything has been diluted by the entertainment shows. Everyone is everywhere four or five times a day. So unless you have a sitting president like Barack Obama on, or a big scandal, you’re better off with a set that helps.”

The new “Tonight Show” set has the usual elements: band shell stage left, desk stage right and a mesh curtain made of metal in the center. But the curtain is curved and, like the rest of the set, is deco in style. It evokes the mood of 30 Rockefeller Plaza but is much grander than O’Brien’s former set in Studio 6A. There are nearly double the number of seats, and there are many more lights flashing. “I liked the set,” Lorne Michaels told me later, “but I wonder if it’s the movie version of a talk show.”

As he wandered around the stage, O’Brien looked comfortable, at home. “The hardest thing in L.A. is a destination,” he said. “I’m spoiled. I’ve had the destination in New York: Rockefeller Center. It’s the high temple of American TV, and we were smack-dab in the middle of it. L.A. is different. Everything can feel like the lunar landscape out here.”

He paced around the set. The set wasn’t finished: there were at least 25 workers installing lights or checking plans or hammering. “I’m happy when I’m in the studio,” O’Brien said. “I’m eating my meals here now.” O’Brien’s desk wasn’t finished yet, and there were three folding chairs where the guests would be. “I miss doing the show,” he said. “I miss the audience. The other day, I was at my daughter’s school to read to the kids, and I started acting out all the characters. I ran into Liza on my way out, and she said, ‘How did it go?’ I said, ‘I killed in there!’ She looked at me and said: ‘They’re a bunch of 4-year-olds. You’re just supposed to read the book.’ ”

O’Brien laughed. He walked over to an X that had been taped to the floor. When the show starts, that will be where he’ll stand when he emerges from the curtain to address the audience. “I come here at night, after everyone’s gone home, and I practice giving the monologue,” he said. “Every night. For hours. I just stand on the X and imagine the rest.”