For years, Letterman was the heir-apparent to his idol, Johnny Carson. But when Carson retired in 1992, the chair went to Jay Leno instead. Letterman, deeply and publicly wounded, moved to CBS.
"I've never considered myself to be a bitter person. When Johnny Carson retired and I was not given the job as host of the tonight show, I was disappointed, but - my way of thinking, it was not bitter," Letterman said during a press conference.
He turned "The Late Show" into a comedic laboratory.
There were top 10 lists and stupid pet tricks.
Television's most reliably irreverent personality is feeling kind of nostalgic these days.
"Now I am told by some staff that you're seen in, not odd but unique places around the Ed Sullivan Theater, as if you're kind of trying to soak it all in," I put forth.
"Well, that's absolutely correct. So when I'm down here during band numbers or during commercial breaks, I will go to various places and -- and try to memorize what it looks like and how I feel, and look at the audience and -- and get the scale of things because even though I've done it for so long, I -- I don't ever wanna be without a fairly accurate, fairly vivid impression of this experience."
By any measure, David Letterman took his craft to new heights. His comic genius recognized by everyone ... except himself.
"I had this conversation as recently as last evening with my wife. And she will go to this strategy to pep me up. ... She says, 'Everything's fine and you've -- you've accomplished some things and you should be proud of that.' And -- and I don't believe her. And so we, you know, we have to be separated. We go -- we go to neutral corners."
"Life can be hard work for you," I said.
"Well, for anybody for God's sakes. Isn't it really?"
Letterman's time in the late night spotlight has had its share of good times and bad times -- some of his own creation.
"You've talked about alcoholism. ... Is that for real," I asked. "I mean, you're not just talking about 'I drink a lot [or] drank a lot.' But you were or are an alcoholic?"
"I guess you -- you're always an alcoholic, yeah."
Letterman said he started drinking when he was about 11 years old.
"It was that old thing where my dad used to like scotch and soda.And, 'Here, here, Dave, you wanna try one?' And I tried one. And I just thought, 'This is fantastic.' ... It was delightful. I -- I just -- I loved everything about it."
"But then in high school it-- it was part of the culture. And in college, it was mandatory. And then when-- when you get outta college people start to taper off. And I was surprised. I would look around and [ask] 'Where are all my drunk buddies?' And you had to go looking for a drunk buddy here and there. They weren't prevalent the way they were in school. And I drank right through 'til I was 34. And I had the -- the show at NBC and I just said to myself, 'You're- - you're a fool, you're a dumb fool. You can't do this. You -- you know, they just don't give these shows to everybody. You have one. And-- and you drink yourself into trouble, you're done, pal." And I just quit. Never, never took another drink."
"It was huge -- because I'd be dead. I -- I'd just be dead."
Remarkable candor for a man so famously private, though Letterman's personal life HAS sometimes become conspicuously public.
In 2009, he admitted to having liaisons with staffers, after a CBS News producer tried to extort money from him.
Letterman delivered an on-air apology. The revelation came just months after his marriage to long-time girlfriend Regina Lasko.
Letterman is no stranger to headlines, as when he underwent quintuple bypass surgery in 2000.
"I don't know anybody who has had open heart surgery loved it."
"It was great because it was all about me. Oh my God it was great," Letterman said. "And people would come in and -- and they would worry about me and they would help me outta bed and they would walk me around the wing of the hospital. And then after I got out of the hospital they would come up to the house and -- it was delightful."
He celebrated his doctors on the air and admits his surgery did have a surprising emotional impact.
"I did get weepy, which I -- I don't think was depression. ... It was a joyful weepiness. You know, I would hear a certain song or a certain image or, you know, be talking to my wife and I would just explode into tears. But it was never that -- that, like, clinical depression where it's, like, I can't get out of bed. 'Oh my God, I can't get out of bed.' It was not that.
"Have you ever had that?" I asked.
"Yes. Yeah, I've had that. And that's just -- that's the worst. That's really the worst. It's -- it's a bottomless pit that sucks you and keeps you suckin' you down. And I -- I never -- I'm glad I went through it because now I understand it."
"One episode or more episodes?"
"One. One ... it lasted about six months," he said.
It was triggered, he said, when he went off prescription medication after a painful outbreak of shingles -- which forced him off the air for more than a month.
Just recently, Letterman found himself a bit player in Brian Williams' woes. The NBC news anchor was caught embellishing the truth in a story he told on Letterman's show about a helicopter mission in Iraq.
Typically, Letterman's version begins with a joke.
"I went to his dressing room and I said, 'You know, that helicopter story, it would be so much better if you mentioned you were in the -- chopper that took the hit," he said, laughing.
Williams had been a frequent guest on late-night talk shows and reportedly even lobbied to host one.
"He would've been fantastic. He would've been great. He's ... a natural born broadcaster."
"You can see the future as well as I can only maybe a little better 'cause you're taller. Do you think he'll end up in comedy?" I asked.
"No. No, I don't think so. I think this will sort itself out. I don't know how, but I think it will -- in a year or two it will be a dim memory."
Letterman, meanwhile, is looking forward to spending more time with his family.
As his audience knows well, his heart belongs to 11-year-old Harry, born when Letterman was 56.
But first he has to say goodbye.
"You had me on during an important point in my life, when I was leaving 'The Today Show,' which was '89-ish. It was probably the most vivid year of my life, and I was a guest on your show. And now you're having what I'm guessing is a profoundly vivid moment in your life. And I'm grateful that you're sharing with it us."
"Well, thank you very much. And I ... I'm naked and afraid because -- and I, it's so cliché, but I'll share it with you anyway -- any enormous uprooting change in my life has petrified me. Really petrified me. But once I've come through the other side, the -- the reward has been unimaginable."
On Wednesday, David Letterman will walk out the stage door for the last time. And it's not clear this time what's on the "other side."
"I notice how when you talk about it you say that you are retiring from the show. ... Retiring form the show is not the same thing as 'I'm retiring."
Right. Yeah, I -- I think I'm trying to make it more palatable to myself. But I -- I doubt that anybody will ever see me again," he said before breaking into a grin and laugh. "Yeah."
I doubt that.