Editor's note: Watch the interview with David Letterman on Tuesday night. "Piers Morgan Tonight" airs at 9 ET weeknights on CNN.
(CNN) -- When Regis Philbin asked David Letterman about the difficult decision to return to the air less than a week after 9/11, the "Late Show" host said, "I remember not wanting to go back, not feeling ready to go back, but knowing we had to go back. And you know -- my concerns were minimal compared to people who really suffered."
Philbin, sitting in for Piers Morgan, in his return to hosting after leaving "Regis and Kelly" gets Letterman to open up about fatherhood, politics, his career, his friendship with the late Johnny Carson and more.
Letterman's "Late Show" guests on September 17, 2001, were Philbin and Dan Rather, and his completely joke-free monologue was solemnly delivered from behind his desk rather than the usual center stage. Letterman praised New York firefighters, police officers and Mayor Rudolph Giuliani. Of those who carried out the attacks, he said: "We were told that they were zealots fueled by religious fervor, and if you live to be a thousand years old, will that make any sense to you?"
When Philbin asked Letterman to take him back to the beginning of his career, Letterman recalled falling behind in college until the semester he took a public speaking course at Ball State University in Muncie, Indiana. He referred to the class as a "lifeline" that made him realize he "might be able to turn this into doing something."
Letterman graduated from Ball State in 1969 and six years later went to California to try to make it as a writer. He found success quickly, he told Philbin, "not because of me. It was just in those days ... if you wanted to go to California and become a comic or get involved in comedy, writing, performing, whatever, the blueprint for that was laid out in front of you every night on 'The Tonight Show.' "
The show's host, Carson, often plugged comedians' appearances at the Comedy Store, and Letterman saw the famed Hollywood comedy club as "an instant connection."
Three years later, in 1978, Letterman appeared on "The Tonight Show."
On that appearance, Carson predicted great success for the young comedian, and Letterman, watching the old "Tonight Show" clip, told Philbin, "That breaks my heart -- to see Johnny again."
Letterman recalled Carson's early influence on him.
"He was, I think, the biggest star in television. And I was just a kid who has followed ... the beacon of his light coming out of Burbank."
Letterman called Carson's friendship "a tremendous blessing" and said he made a point of telling him the way he felt. Letterman recalled the last time he saw Carson.
"The way life is," Letterman said, "you don't know that that will be the last time, but it turned out to be the last time, and it couldn't have been a lovelier evening. And I cherish that because it was unusual. It was not going to happen under any other circumstances. And it was my wife, myself, Johnny and his wife on Johnny's yacht that he had anchored in the Hudson.
"And it was a Friday evening and we sailed off just before sunset and went up the Hudson, up under the George Washington Bridge, which is lovely, turned around, now the sun is setting. We go out to the Statue of Liberty and see that at night, as the sky is darkening. And then you turn around and we headed up the East River, and you see the lower tip of Manhattan , and it was a sight and an experience. ... You never get to see New York like that."
Carson passed away in 2005.
Letterman has hosted "Late Show With David Letterman" on CBS for nearly 19 years. Before then, he hosted "Late Night With David Letterman" on NBC for 11 years.
One staple of both shows -- the "Top Ten" list -- is still going strong.
"It was a cheap, easy way to refill a category and get some laughs most nights," Letterman said. When Philbin asked if coming up with the lists was "a chore," the "Late Show" host joked, "Not for me. ... I'm not even in the building. I'm an old man with a facial."
Letterman also said that his favorite guests -- besides Philbin, of course -- were "people who really come and do a great job for us." He counted Tom Hanks, Bill Murray, Brian Williams, Tom Brokaw and Don Rickles among the "handful of people who are really, really strong."
When Philbin brought up the fact that, in the past, talk-show hosts have avoided politics so as not to alienate their audiences, Letterman said, "I have been guilty of appearing to be playing partisan politics. However, I'd just like to say that for the record, I am a registered independent."
Letterman insisted the material drives his jokes, and noted that "no president that I am aware of got hammered harder than Bill -- President Bill Clinton -- over the Monica Lewinsky situation. We beat up on him. We still use him as a reference, and then we were desperate. We thought, 'Well, this was so easy' -- and then we got George Bush."
Letterman said that while he may appear to have a political agenda, it's simply a matter of who's easier to poke fun at.
"If a guy drops his dog or a guy straps his dog to the roof of a car or if a guy gets a shoe thrown at him, well, this is where the material is going to be," Letterman said.
Philbin also asked Letterman about fatherhood. The comedian's son, Harry, is 8.
"Life is no longer solely about you," Letterman said. "It's about him."
Philbin asked if Harry knew what his dad did for a living.
"No," the comedian joked, "he thinks I have a job in a metal shop."
Letterman said he and his son took up skiing four years ago and very much enjoy the sport. Harry also loves animals, Letterman told Philbin, and likes to visit his father at work when "Late Show" does its "Stupid Pet Tricks" segment or has zookeeper Jack Hanna as a guest.
When Philbin asked whether Harry was a "mama's boy" or a "daddy's boy," Letterman answered, "He's a mama's boy, for sure."
What's great about being the 65-year-old father of an 8-year-old?
"I'm old enough now where if I see trouble down the road, I'll probably be dead by then, and it will be his stepfather's problem."
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