Unlike Letterman, Leno and O'Brien do not own their shows and cannot make similar arrangements. Comedy Central's Jon Stewart and Stephen Colbert are also unable to make separate deals with the WGA because their programs are owned by their networks. On Monday, a Comedy Central spokesman said it was unclear when they would return to the air.
Debbie Vickers, executive producer of "The Tonight Show," said the slumping viewership wasn't the main factor behind the hosts' decision to return.
"I don't think it was the driving force," she said. "These guys want to do shows."
Jeff Ross, executive producer of "Late Night," said, "We put it off as long as we could."
NBC will begin paying the salaries of some producers and bookers this week as the shows ramp up, with Leno and O'Brien covering the rest of the payroll until the end of the year.
In announcing their return, the network compared Leno and O'Brien to Johnny Carson, who returned to the air without his writers during the 1988 writers strike. However, unlike his NBC successors, Carson was not a guild member and was able to perform his own material.
As members of the union, Leno and O'Brien face a trickier challenge. Under WGA strike rules, they are prohibited from writing material that otherwise would have been penned by guild writers -- including themselves.
"If they are employed as writers on the show, the rule as we see it is that they are barred from performing writing services," said Tony Segall, general counsel of WGA West.
That stance could rankle NBC executives, who said they hoped Leno and O'Brien would be able to write their own jokes.
O'Brien pledged to talk up the writers' cause on the air, adding that without his staff, "my show will not be as good. In fact, in moments it may very well be terrible."
The two programs will probably try to add more guest interviews to fill time, a prospect that presents its own hurdles because many actors have sided publicly with the WGA and could be reluctant to cross the picket line to appear on the program.
But some Hollywood veterans said they believed that many actors would have no problem appearing on the shows.
"You're going to find very few stars who are not going to go on," publicist Howard Bragman said.
Producers of both shows said they had heard similar sentiments recently from representatives of prospective guests.
"It does seem like people are warming to the idea," Vickers of "The Tonight Show" said. "January feels better than December did."
"I think there is some strike fatigue," she added. "You want to give everyone the opportunity to have resolution. But if there aren't resolutions, we can't just wait and wait and wait."
However, getting prominent actors to walk the red carpet at the Globes and the Oscars could be more difficult, if the past is any indication. In 1980, the Emmy Awards were held during a Screen Actors Guild strike. In a show of support for their union, 51 of the 52 nominated performers boycotted the event.
"I don't think there's anybody involved who's nominated who won't have this on his mind in a major way even if turns out there is no picket line," David Cronenberg, director of "Eastern Promises," said last week after being nominated for a Golden Globe. "It'd be hard to celebrate full blast given the way people are hurting. It makes it a difficult situation."
Times staff writers Rachel Abramowitz, John Horn, Chris Lee and Meg James contributed to this report.