George Clooney handcuffs himself to host David Letterman on the set of the “Late Show.

 George Clooney handcuffs himself to host David Letterman on the set of the “Late Show.

Jeffrey R. Staab/AP

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It was a typical week for late-night television: On NBC's Tonight show, the host, Jimmy Fallon, played a catchphrase guessing game with the Modern Family star Sofia Vergara, and re-enacted the music video for the 1990 rock ballad "More Than Words."

On ABC's Jimmy Kimmel Live, Jimmy Kimmel watched a hidden camera to see how Los Angeles pedestrians would react to a burrito dangled above Hollywood Boulevard. And on CBS' Late Late Show, James Corden asked pizzeria customers to choose between the pies they had ordered and the contents of a mystery box.

Elsewhere, on CBS' Late Show, David Letterman was interviewing President Barack Obama - a ruminative, sometimes funny, casual conversation between two men who seemed almost like peers, joking about how they might share a game of dominoes after they retire.

Letterman, of course, will make that transition first: He is ending his 22-year run on Late Show (and a 33-year late night career) on Wednesday.

When he walks out of the famously chilly Ed Sullivan Theater for the last time, he will be leaving a late-night landscape that while brimming with potential and still attracting millions of viewers, is also scrambling to redefine itself in a world where late-night TV shows are increasingly not being watched at night, not for more than a few minutes and not on a TV.

If Letterman represented an era when a late-night show was a comprehensive end-of-day viewing experience, meant to be watched in a post-twilight setting for a full hour (or until you fell asleep), the coming age is fragmented by technology, designed for online virality, unstructured and unmoored from any time slots.

Jimmy Kimmel

Jimmy Kimmel

Richard Cartwright/AP

"People are just plucking your greatest hits, without having to sit through the rest of the show," said Kimmel, 47, the ABC host. "There's more focus on singles than on albums."

Many of the classic trappings of late-night shows are still visible: opening monologues, house bands and desk-side celebrity chitchats. What is going away is the expectation that viewers will watch these programs in close to their entirety, or even sequentially.

And future shows will abandon the familiar, rhythmic tempo of late night altogether. Next year, Chelsea Handler, the former host of the E! series Chelsea Lately, plans to introduce a Netflix series that can be watched at any time of day and whose contents could vary widely from episode to episode. (Handler, like Trevor Noah, who will replace Jon Stewart on The Daily Show, has yet to appear on the air in a new late-night hosting role.)

In the meantime, the people currently creating late-night television are experiencing varying degrees of head-scratching and hand-wringing and periods of Internet sniping as they try to determine how to distinguish their programs. In a positive development for these shows, the Internet has allowed them to reach larger audiences while their broadcast ratings remain healthy.

Fallon, 40, the 10:35 p.m. champion, has nearly 4 million nightly viewers, while Kimmel and Letterman get about 2.7 million each. (These numbers are down overall from earlier, less fragmented eras - Jay Leno's Tonight show drew about 6 million viewers and Letterman's Late Show about 4 million when they went head-to-head in the 2000s.)

Jimmy Fallon on the set of "The Tonight Show Starring Jimmy Fallon"

Jimmy Fallon on the set of "The Tonight Show Starring Jimmy Fallon"

Lloyd Bishop/AP

A successful online video from one of those shows, however, can be even more powerful. A reunion of the Saved by the Bell cast or a woman appearing to set herself aflame while attempting to twerk can receive tens of millions of views.

Fallon's recurring celebrity lip-sync competitions are popular enough to have spawned a hit spinoff series, Lip Sync Battle, on the Spike cable channel.

Still, there is concern that pursuing online virality for its own sake will backfire. "You cannot be chasing that, because it's futile," said Corden, 36, the British actor who took over Late Late Show in March.

"If you just want to have a great viral clip," he added, "I'll strip naked and run down Sunset Boulevard. That will get loads of views."

Stephen Colbert, 50, who played an acerbic political commentator on The Colbert Report on Comedy Central, will set aside this character when he takes over at Late Show in September.

Letterman, whose Late Night program on NBC provided a cutting-edge companion to Johnny Carson's Tonight show in the 1980s, was welcoming of this latest wave of change.

Letterman said the seeds of change "were sown decades ago, and good for these guys, that they found something else to do."

Merrill Markoe, Letterman's original head writer at Late Night, said that many of its comic creations - top 10 lists; Velcro suits; hydraulic-press experiments - were a response at the time to the perceived rigidity of Carson's show. The challenge she and her colleagues faced, she said, was to "bring something new to the landscape," which she said is "still the job definition."

"Over 20 years ago, we said goodbye to Carson," said Nell Scovell, a former writer for Letterman's Late Night program.

"This month, we'll say goodbye to Letterman," she added. "And 20 years from now, we'll all lip-sync goodbye to Jimmy Fallon."

Dave Itzikoff, The New York Times

Finale week lineup

Monday: Tom Hanks, with Eddie Vedder as musical guest; Tuesday: Bill Murray; Wednesday: Surprises, highlights and the final Top 10.

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