Like other competitors, Mr. Fallon professed unabashed awe that Mr. Colbert could sustain this performance at such a high level for so long. “Before he won the Emmy, I had been preaching that people had to recognize what he was doing: He’s faking a person,” Mr. Fallon said. “I was one of those who said, ‘He’ll do it for six months and then he’ll move on.’ Imagine if you were still trying to do the Coneheads on ‘Saturday Night Live.’ It’s gets old. But not this. He’s a genius.”

But for all the effort and talent Mr. Colbert has invested in keeping the character going, he appears resolute in his intention to allow him to pass into comic history. This decision, Mr. O’Brien said, reflects tremendous confidence. “There is something really cool about Stephen saying, ‘This needs to stop at some point, so let’s stop now,’ ” he said.

In some cases, a classic comic creation defines a performer for life. Jack Benny was long associated with being a cheapskate, butchering his violin playing and lying about his age. Classic comic characters are often too fully realized to abandon.

Mr. O’Brien said that hanging on to a persona was an American convention, while British performers seem willing to walk away from their signature creations. He cited John Cleese’s Basil Fawlty and Steve Coogan’s Alan Partridge (a narcissistic media personality), though Mr. Coogan has resurrected Partridge a few times.

Ricky Gervais has also recently revived David Brent, the bumbling manager he portrayed on the British version of “The Office,” in live concert performances. “I did consider not bringing him back to life,” Mr. Gervais said. But in a new context — Brent as would-be rock star — the character could be seen as realistically having moved on.

“What’s interesting about what Colbert is doing,” Mr. Gervais said in an interview, “what’s brave and possibly confusing about it, is that he’s always used his own name.” He added: “It can be a dangerous game to play. He presented himself as the character, and now he’s not going to be it. And people are going to miss the character. It’s a joy to watch someone do a really great parody with a character saying the opposite of what you know is right.”