Good Night to Late-Night Guest Hosts : Television: In the Carson era, substitutes regularly sat in. Although ‘Nightline’ sometimes goes without Ted Koppel, the fight between David Letterman and Jay Leno is head-to-head.

Once upon a time, guest hosts thrived on late-night TV, namely in the Johnny Carson years of “The Tonight Show.”

That era is dead as CBS’ David Letterman and NBC’s Jay Leno go at each other night after night in the early stages of their competition. Letterman, in fact, never had a guest host in his old NBC series either.

But in the Carson years at “Tonight,” the many substitutes included Joey Bishop, Joan Rivers, Bob Newhart, David Brenner, John Davidson, McLean Stevenson, Jerry Lewis, Bill Cosby, Garry Shandling--and, oh yes, Letterman and Leno.

Almost as dead as guest hosts is the role that past “Tonight” regulars established--that of sidekick/announcer/straight man, as epitomized by Hugh Downs with Jack Paar and, above all, Ed McMahon with Carson.

Arsenio Hall was a loner too in his recently completed late-night series.

But Conan O’Brien has indeed enlisted a sidekick, Andy Richter. Whatever you think of Richter, the move makes sense because O’Brien needs all the help he can get and shows few signs of being able to carry the show.

Of all places, just about the only late-hour network series where you can regularly find guest hosts these days is ABC’s “Nightline,” where, on any given evening, you may find Cokie Roberts, Chris Wallace, Barbara Walters or other correspondents sitting in for Ted Koppel.

And, the truth is, it’s often refreshing and a relief for viewers, just as a day or two off now and then is probably good to revitalize hosts like Koppel--and Leno and Letterman--from the killer grind of trying to be super-sharp on every single outing, which is impossible.

Carson’s “Tonight Show” was a breeding ground for guest hosts who went on to their own programs, among them Rivers and Leno--who both were premier substitute hosts on the NBC series.

Rivers, who wanted to take over “Tonight” when Carson retired, left the show earlier in a bitter separation from the star. And Leno, of course, used the series as a springboard to beat out Letterman as Carson’s heir.

Even “Nightline” is a star-maker, with Forrest Sawyer using his substitute role--and high-profile reporting during the Persian Gulf War--to build his reputation, which resulted in a top position on the prime-time newsmagazine series “Day One.”

Cokie Roberts also seems destined for bigger things at ABC, in no small part to her frequent “Nightline” outings.

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Richter is more of a comic foil on the O’Brien show. Perhaps because of generational differences, there is not likely to be another McMahon, who was caricatured by younger comedians but was an intrinsic part of the “Tonight” rituals and success.

McMahon once said of his role: “My job is to stand back when he (Carson) needs me to--and to make him look good.”

A good sidekick can be a great safety net, a haven for a star host when he’s suddenly at loose ends and doesn’t know where to turn. Carson not only had McMahon but bandleader Doc Severinsen as well.

Letterman and Leno have combined the sidekick-bandleader roles into a single person they can play off of, or at least are trying to. Letterman’s foil is bandleader Paul Shaffer, a genuinely cool TV personality who may be worth half a rating point all by himself in the show’s success.

Leno, meanwhile, seems to be relying more and more on his own bandleader, Branford Marsalis, who is not nearly as exuberant or as much of a TV natural as Shaffer. Still, Leno appears to be comfortable with him at his side and is betting that their rapport will pay dividends for “Tonight” in the long run.

As for the current disappearance of guest hosts, no one knows the late-night terrain much better than Peter Lassally, formerly of “Tonight” and now co-executive producer of Letterman’s CBS series. Says Lassally:

“The why of the matter is that Johnny Carson had been there a long time and started the guest-host game because the longer he was there, the less he wanted to work, and that was the only solution. With Jay Leno and David Letterman starting fresh programs, they felt they had to do five days a week to win against each other.

“So I don’t think they’ve reached the point where they want to do three or four days a week. But I think the day will come when they will consider guest hosts. It’s very wearing to do five nights a week, and I think that both David and Jay will reach the point where that’s an issue.”

As for the sidekick change, Lassally adds: “I’ve noticed that Jay is using Branford more and more just for reaction, which is the way Dave uses Paul. It’s a great device for the host to make side comments rather than making them to the person at home.”

Despite the fact that “Nightline” seems constantly refreshed by its guest hosts--and is doing very well in the ratings--CBS Entertainment President Peter Tortorici thinks it is important for Letterman to be on every day.

“In our situation,” he says, “the most important thing we can do is keep his face and personality on the air in every possible way. We’re just establishing his identity on our network.”

Letterman’s constant presence--cool and hip, just the opposite of CBS’ image--becomes even more important to the network as it appears to be facing increasing programming problems and competition from youth-oriented ABC.

Rod Perth, late-night vice president for CBS, thinks that there’s also a “philosophical issue” because Letterman’s past and present series have been so identified with the star’s offbeat attitude that “in effect, a guest host was inappropriate.”

In addition, he says--citing cable--”there are so many options now for emerging talk hosts. Others who used to be guests now can get their own shows. There are talk networks. There’s Comedy Central.”

Rick Ludwin, NBC late-night vice president, concurs that Letterman’s “distinctive style” meant that “anyone who stepped into that role would have been perceived as not being as good. And Jay has an incredible work ethic. If the staff could do it, he’d do 52 weeks a year and probably do ‘NBC News at Sunrise’ too.”

Ludwin adds that while Carson’s “Tonight” had some top-notch substitutes, “in the ‘70s there was a parade of guest hosts who probably drove more people away than they attracted. Because of today’s competitive environment, that luxury is no longer an option. There are other choices. You can’t afford not to be at your best every single night.”

So the pressure is on. The best thing about the Carson “Tonight” show was the leisurely first 20 minutes or so--the monologue followed by the chat with McMahon that led to ritual comedy bits. The celebrity guests that followed invariably slowed things down. They still do.