CreditDamon Winter/The New York Times

In a single bound, David Letterman seemed to leap the full length of the stage at the Ed Sullivan Theater, racing from backstage as if he’d been thrust forward by the fanfare played by his longtime bandleader, Paul Shaffer, and his CBS Orchestra, and by the rumble of his announcer, Alan Kalter, bellowing his name — “Daaaaay-vid Leh-terrrr-maaaaaaaan!”

It was a routine Mr. Letterman, 68, has performed countless times but will repeat no more after May 20, when he will preside over his last episode of “Late Show,” the CBS franchise he established and has hosted since 1993. Like the veteran slugger who comes to the ballpark for batting practice, he was here on this April afternoon partly to warm up his swing on a few easy pitches, but mostly to put on a show.

No home viewers were watching as he twirled his microphone around like a Wild West lasso, walked it across the floor like a dog and leaned on an expensive broadcast camera. This was a pretaping ritual Mr. Letterman was doing only for the few hundred audience members in the theater. Or maybe he was doing it only for himself.

Photo

David Letterman dressing for a taping of “Late Show.” Each cup to the left represents a completed show. Credit Damon Winter/The New York Times

“Everything O.K. at home?” he asked the crowd. “Everything O.K. at work?” Met with mostly cheers, he laughed and added: “You don’t find yourself filled with some kind of emotional longing? Are we emotionally stable?”

But how could these fans not be riddled with angst, knowing that in a few weeks, Mr. Letterman would bid a heartfelt good night to all of this, after a run of more than 33 years in late-night television — even longer than the three-decade tenure of his mentor, Johnny Carson. After that last show, he will head home to his wife, Regina, and 11-year-old son, Harry, and try to figure out what comes next.

Late-night television will feel the loss of Mr. Letterman, one of its most innovative and unpredictable broadcasters, who in 1982 took a sleepy NBC time slot following Carson’s “Tonight” show and transformed it into a ceaseless engine for Top 10 Lists, Stupid Pet Tricks and a decade’s worth of pioneering comedy bits.

With almost no blueprint to follow, Mr. Letterman showed that late-night TV could offer more than a what’s-in-the-news monologue and witty banter with celebrity guests (though he was capable of doing all that, as well). He made his show a home for misfits and oddballs, for Andy Kaufman’s slap fights and Larry (Bud) Melman’s shrill soliloquies, where champion bird callers or his own mother were deemed as important as Hollywood ingénues or rising rock bands.

Mr. Letterman proved he could reinvent himself, too: When he was passed over as Mr. Carson’s heir in favor of Jay Leno, he packed up for the uncharted territory of CBS and became a more inclusive — if still idiosyncratic — master of ceremonies.

But Mr. Letterman is leaving a late-night biosphere very different from the one he helped thrive. Hosts like Jimmy Fallon (who ultimately replaced Mr. Leno at “Tonight”) and Jimmy Kimmel (at ABC’s “Jimmy Kimmel Live”) are dominating with their own ingenious energy, their Internet savvy and their visible youth, and Mr. Letterman is about to be replaced by Stephen Colbert, the politically astute smart aleck of “The Colbert Report.”

Not that any of these issues appeared to be on Mr. Letterman’s mind during his preshow set. Asked by an audience member from Newberg, Ore., if he had any advice for that city’s impending graduates, Mr. Letterman replied, “Treat a lady like a whore, and a whore like a lady.” After some laughter at this seemingly un-Letterman-like joke, the host chuckled to himself and said: “I don’t know why I would say something like that.”

“What do you care?” Mr. Shaffer said.

But no matter how hard he has tried to hide it over the years, Mr. Letterman does care. As he said, more sincerely, to the man who had asked for graduation advice, “If you do good things for people, it will never stop making you feel good about yourself.”

Upstairs in his “Late Show” offices a few hours later, a contemplative Mr. Letterman emerged, dressed in khakis and a T-shirt that said “Genetically Engineered Trout Is Safe!” to reflect on all that he has learned along the way. In these edited excerpts from that conversation, he offered his unguarded and unsparing assessments of his heroes, his colleagues, his would-be successors and himself.

FOR THE QUESTIONS AND ANSWERS IN THE LETTERMAN INTERVIEW, PLEASE CLICK  HERE