LOS ANGELES  Jay Leno, who has lived a life of comedy, knows the rules as well as anybody who has ever stood on a stage and told jokes. “The first rule of comedy is: Don’t create anything bigger than yourself,” Mr. Leno said in an interview in the green room of “The Tonight Show.” “The trick is to keep the water just below a rolling boil so it stays in the pot a little bit longer.”

But the truth is that the comedy pot where Mr. Leno and his many competitors live  late-night television  has been simmering ever since Mr. Leno stunned the television business in 2004 by agreeing to step down from the host role where he has thrived since 1992 to make room at “The Tonight Show” for NBC’s other late-night star, Conan O’Brien, currently the host of “Late Night.”

And until last week it seemed that the pot was about to spill over into a new round of overheated competition, with Mr. Leno moving toward accepting an offer from ABC to star in a show to challenge both Mr. O’Brien and David Letterman on CBS. The deal was close enough to coming to fruition that ABC affiliates were getting word to be ready for a change to Mr. Leno in January 2010, according to what NBC was hearing from its own affiliates.

Then Mr. Leno shocked the television world again by agreeing to stay at NBC to create a new version of his show, tentatively titled “The Jay Leno Show,” at 10 every weeknight starting next September, moving from late night to prime time.