David Letterman rarely played it neutral like other late-night talk-show hosts. If he liked you, he'd let you know. If he didn't like you, he'd let you know that too, but probably in a more subtle way. (Letterman, for the most part, was always a professional.) And if he loved you, he made sure you were back on his show again and again.

David Letterman loved Warren Zevon.

More so than any other musical guest Letterman -- who's retiring this week -- has had on his two late-night TV shows over the past 33 years, Zevon was a perennial favorite, appearing more than a dozen times. Not just because he wrote and sang some great songs, but because he was funny, self-deprecating and a cynical bastard -- just like Letterman.

Zevon appeared during the first year of NBC's Late Night With David Letterman, which Letterman hosted from 1982-93. And when Zevon was diagnosed in 2002 with the cancer that would eventually kill him, Letterman devoted the entire show (this time CBS' Late Show With David Letterman) to his old friend later in the year, where the two chatted and shared jokes, and Zevon played some of his great songs one last time.

When Letterman first had Zevon on the show -- in July 1982, shortly after the release of his fifth album, The Envoy -- the singer-songwriter's career wasn't exactly at a high point. His self-titled album from 1976 made him a favorite of rock critics and fellow musicians, but few people bought it. That all changed two years later when Excitable Boy reached the Top 10, thanks to the success of the Top 30 hit "Werewolves of London."

But Excitable Boy's follow-up, 1980's Bad Luck Streak in Dancing School, stalled at No. 20 as Zevon recovered from alcohol abuse. And nobody was expecting too much from The Envoy, which barely cracked the Top 100. But Letterman, a fan, didn't care about all that. He cared about the music, and The Envoy was a good, if difficult, album to work through.

Zevon played two songs on his first Letterman appearance -- Excitable Boy's title track and "The Overdraft" from the new LP -- and he sat down to talk with the host, and he proved to be a charming, witty and biting guest, unlike most musicians (which is why they mostly just perform their hit song and get out of there). No doubt Letterman saw something of himself in Zevon, who in turn opened up in ways to Letterman that he rarely did in other interviews.

Watch Warren Zevon's First 'Letterman' Appearance From 1982

It would be another five years before Zevon released another album, 1987's Sentimental Hygiene, which was recorded with three-fourths of R.E.M., a band he struck up a friendship with a few years earlier when they made some music together as Hindu Love Gods. (They eventually cobbled together an album made during an all-night session playing covers, including Prince's "Raspberry Beret.")

The Envoy's disappointing sales led to Zevon's record company dropping him, which led to another period of excessive drinking and drugging. Sentimental Hygiene was touted as a comeback (Zevon would hear this several times over the next decade and a half), with songs like "Boom Boom Mancini" and "Trouble Waiting to Happen" among his best in years.

The album fared a little better than the previous one on the chart, making it to No. 63. But more importantly it marked Zevon's return to music, and Letterman was there with open arms and a stage. He played "Boom Boom Mancini" to promote the album in 1987 and then returned a year later for Letterman's sixth-anniversary show, performing "Lawyers, Guns and Money," which was preceded by a bit of "Trouble," a song Jerry Leiber and Mike Stoller wrote for the 1958 Elvis Presley movie Kid Creole.

Watch Warren Zevon Perform "Trouble" and "Lawyers, Guns and Money" on 'Letterman'

Then came another decade and three albums, none of which charted. But Zevon made nearly a dozen Letterman appearances during this period -- a time marked by Letterman's switch from NBC to CBS. Some times Zevon was there to play a song from his current album, like he did in 1989 with "Splendid Isolation" from Transverse City and in 1995 with "Seminole Bingo" from Mutineer.

Other times he was there to fill in for Paul Shaffer as Letterman's musical director and house band leader. "When it comes to music, there's just a handful of folks that I really, really truly love and adore," Letterman said while introducing Zevon during one of these appearances. "We are lucky to have one of those with us tonight." Ultimately, Zevon took over for Shaffer "about 20 times" according to Letterman's estimation, as in 1998 when Letterman's longtime sidekick was shooting a part for Blues Brothers 2000.

"He was a schooled musician, a former classical pianist and someone who was capable of writing out an entire score," Shaffer tells Ultimate Classic Rock. "People don't realize this about him. He also fulfilled the other requirement of the show. He was able to make Dave feel comfortable. He was one of Dave's favorite recording artists. When he looked over and saw Warren Zevon there, he got a kick out of that. And I could more easily take time off, because I knew Warren had me covered."

Zevon proved to be an affable and capable band leader during these segments, bantering with the host and leading the group through intro and outro songs, as well as performing a few of his own cuts, which sometimes were tied to new albums (like the title track to 1991's Mr. Bad Example), and sometimes weren't. They were always amusing.

Watch Warren Zevon Sit in for Paul Shaffer on 'Letterman'

Even though Zevon's two '90s albums (Mr. Bad Example and Mutineer) and 1987's Transverse City didn't chart, he managed to debut at No. 173 with 2000's Life'll Kill Ya, and once again made the promotional rounds on his friend's show. (Letterman wasn't the only TV host to welcome Zevon during this time; he also showed up on The Late Late Show, Later ... With Jools Holland and other programs from time to time).

But Letterman was the one Zevon regularly returned to and usually went to first. He once again replaced Shaffer as band leader in 2000, and in 2002, he appeared with his son Jordan to sing "Hit Somebody! (The Hockey Song)" from that year's My Ride's Here. (Incidentally, the album version of the song features Shaffer on organ and Letterman on backing vocals during the chorus.)

Listen to Warren Zevon's "Hit Somebody! (The Hockey Song)" with David Letterman

That year, Zevon felt dizzy before a concert and soon developed a cough he couldn't shake. He never liked or trusted doctors. But he eventually relented and visited a physician, who informed him that he had inoperable peritoneal mesothelioma, a form of lung cancer usually associated with asbestos. (Zevon's son later claimed he used to play in the attic of his dad's carpet store when he was young.)

Zevon refused treatments for the cancer that was killing him, and instead rounded up a group of famous friends (including Jackson Browne, Don Henley, Tom Petty, Bruce Springsteen and others) to record one last album, The Wind, which came out 10 days before his death on Sept. 7, 2003, at the age of 56.

Eleven months before, on Oct. 30, 2002, Zevon appeared on Letterman's show one last time. He was given the entire hour to talk about his diagnosis, his life, his music and whatever came to mind during the chat. It's one of late-night television's most poignant moments, but not one without humor and insight.

"First of all, let me say that I might have made a tactical error in not going to a physician for 20 years," Zevon told Letterman when asked how he found out about the cancer. "It was one of those phobias that really didn't pay off." When Letterman says he's not sure he could make jokes under the circumstances, Zevon assured him, "I know you would." It's a heartbreaking moment, and a riveting hour -- one of Letterman's very best.

Watch Warren Zevon's Last Appearance on 'Letterman'

And nobody but an old friend could make it work. Zevon opens up, laughs and shares stories from his past. Letterman encourages, holds back and shows the sort of empathy rarely exhibited by television hosts. He lets Zevon progress at his own pace, and it's a remarkably brisk one. "I don't feel as bad as they say I am," Zevon tells Letterman at one point.

When asked how the diagnosis has affected his work and outlook, Zevon says, "You put more value in every minute. ... It's more valuable now. You're reminded to enjoy every sandwich."

Letterman never loses control of the show or the talk or his emotions, but there are times where you can sense his ache in hearing his old friend come to terms with the end of his life. Along with The Wind's closing number and centerpiece, "Keep Me in Your Heart," it's the most moving moment of Zevon's last year, and one that Letterman handled perfectly.

Zevon performed three songs, all while sitting and playing the piano, during his last appearance: "Mutineer," "Genius" and "Roland the Headless Thompson Gunner." Like the rest of that hour-long show devoted to Zevon, the performances serve as a plaintive reminder of his often-overlooked and underappreciated gifts as a storyteller, singer-songwriter and artist. Before he left the studio for the last time, Zevon handed Letterman his guitar. "Take care of this for me," he said.

Letterman opened Zevon's soul that night, and in turn, Zevon made Letterman a little more human. It's a watershed moment of late-night TV, and one we'll likely never see again.

"He performed three songs ... and rehearsed each of them with our band, including one that had a string quartet [that he] arranged," Shaffer recalls. "This is a big workload for anyone, let alone somebody who was dying. But he was so happy to be working. That afternoon, as we rehearsed those three songs, even though I said, 'Warren, Just try to mark it and don't blow your voice,' he couldn't help it. It was so much fun playing that afternoon. He was a little more tired in the evening, as anybody would be, but especially somebody as sick like he was. But I remember those rehearsals were amazing."

The day after Zevon's death, Letterman talked about his old friend for 10 minutes on air, and he opened up about what drew him in. "The music itself was just exciting," Letterman said. "It was just thundering and exciting and rhythmic and complicated. ... It was not the kind of rock 'n' roll you'd hear much of. And then the lyrics ... were so vivid. Each song was like watching a motion picture. He was a poet and a storyteller and a good friend of ours."

Watch Warren Zevon Perform 'Roland rhe Headless Thompson Gunner' A Final Time

See the Yearbook Photos of Warren Zevon and Other Rock Stars

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