David Letterman interviews Ukraine's Zelenskiy in underground subway

In October, the late-night host traveled to Kiev to interview Ukrainian President Volodymyr Zelenskiy in the safest part of the city — a subway platform 300 feet (90 meters) below ground.

A small audience watched in the semi-dark as subway trains rumbled past. At one point a siren went off. Letterman looked alarmed, but no one else reacted. What did that indicate, he asked?

"Unfortunately it means that war has become a habit," said Zelenskiy. "Many Ukrainians have gotten used to it... To me sirens are an indicator that war is not over... this is a reminder that somewhere, someone is giving up their life for yours."

In February, Russia sent troops into Ukraine in what it called a "special operation" to degrade its southern neighbor's military capabilities and root out people it calls dangerous nationalists. Ukraine and the West say the war is an unprovoked act of aggression.

Letterman also spoke with the head of Ukraine's railways - vital to the war effort - and visited a comedy club. He discussed with Zelenskiy, a former comedian, the importance of humor in dark times, and chatted with him about his family.

It was only in speaking to his wife and children that he could breathe, said the Ukrainian leader.

"Children know much more about the war than we do. My son is 9 years old. He knows the names of all the weapons... Each of us makes sacrifices for our country. Our children sacrificed their childhood," he said.

Letterman's Netflix show "My Next Guest Needs No Introduction" has in previous episodes featured talks with former President Barack Obama, rapper Jay-Z and singer Billie Eilish.

Netflix has been spending heavily to produce and acquire content as it races to dominate streaming television and beat competition from traditional media.