THE MYSTERIOUS ROAD TO SERIES SUCCESS.

Byline: Andy Meisler The New York Times

Was it the star? The format? The writing staff? The golden time slot selected by some astute network mastermind?

Those are some of the questions that percolate when a venerable weekly television series such as CBS' ``Murder, She Wrote,'' which is ending on May 19, lasts a rare and fabulously profitable 12 seasons.

Or was it, concludes the list of questions, some arcane but duplicable combination of all these things? In other words, is there a formula for series longevity, a way to keep successful shows running indefinitely by confounding the built-in forces that cause them to self-destruct?

The somewhat surprising answer is yes.

Not that revealing it would do anyone much good. Since the dawn of the television age, most series, even those conceived and supervised by the medium's best minds, have imploded well before they have amassed the 60-odd episodes needed to reach syndication.

But television history is dotted with long-running series that defied the odds: shows like CBS' ``Gunsmoke'' (20 seasons) and ``Lassie'' (17 seasons) and NBC's ``Perry Como Show'' (15 seasons).

In recent years, the champions have been CBS' ``Dallas'' (14 seasons) and ``MASH'' (11); NBC's ``Cheers'' (11); Fox's ``Married ... With Children'' (still running after 10 seasons) and Fox's ``Simpsons'' and ABC's ``Family Matters'' (both still going after eight).

Some of these shows have been critically acclaimed, some not. What is it that they do have in common?

``They all provide the viewer with a major, major comfort level,'' says Leslie Moonves, president of CBS Entertainment, ``with situations you want to revisit and a star that you want to live with for a long time, with whom you can really feel comfortable in your home.''

Warren Littlefield, Moonves' counterpart at NBC, has another answer: ``Change. That's the No. 1 factor in making a show successful in the long run. Something we forget is that the audience doesn't want to see the same thing every week. They welcome change. It freshens the show's dynamic. It's a critical part of every series' success.''

These two statements seem contradictory, but together they define the creative boundaries of a maddening task: keeping the average viewer from getting bored without sabotaging a comfortable relationship with familiar characters and setting. Looking at each long-running show, one can see clear instances in which its producers successfully achieved this feat.

``Sometimes you have to look closely,'' says Moonves. He points out that ``Murder, She Wrote,'' at first glance virtually unchanged over its long run, went through a number of subtle modifications both in front of the camera and behind it.

The characters surrounding Angela Lansbury's genteel supersleuth Jessica Fletcher - the mayor of Cabot Cove, Maine; the sheriff; and various big-city cops with whom she formed multi-episode alliances - were changed periodically. In 1991, in a move to open up more story possibilities, Jessica relocated from rural Cabot Cove to New York.

For his part, Littlefield points out that ``Cheers'' managed consistently to replace vital components - namely its beloved bartender Coach, played by Nicholas Colasanto, who died during the 1984-85 season, and female co-star Shelley Long, who quit the show for a movie career in 1987 - while developing a strong rotating cast of characters such as psychiatrist Frasier Crane (Kelsey Grammer) and his wife, Lilith Sternin (Bebe Neuwirth).

At the same time, say the two executives, the powers-that-be behind ``Murder, She Wrote'' and those behind ``Cheers'' successfully identified the vital few elements of the shows that could not be changed.

In the case of ``Murder, She Wrote,'' it was Lansbury, who asked for and received the title executive producer midway through the show's run. In the case of ``Cheers,'' the bottom line was drawn at its cozy barroom setting and its star, Ted Danson, who was glued to the show by the largest weekly paycheck in television history, reportedly $450,000 an episode.

Another model for a show's successful retooling is ``Beverly Hills, 90210'' on Fox. Still going strong after six seasons, the show has survived the loss of top stars like Luke Perry and Shannen Doherty.

What it could not survive, decided the executive producer, Aaron Spelling, was having its cast of post-teen-age actors frozen in its original high-school setting.

CAPTION(S):

5 Photos

Photo: (1) Lassie's owners - including Jon Provost and J une Lockhart - came and went, but the pup herself stayed on CBS for 17 seasons.

(2) They may only be cartoons, but ``The Simpsons'' have been amusing Fox audiences for eight seasons.

(3) The undisputed series champ is ``Gunsmoke,'' which ran 20 seasons on CBS and made household names of Milburn Stone, Amanda Blake, James Arness and Dennis Weaver.

(4) She moved out of small-town Cabot Cove in 1991, but Angela Lansbury's Jessica Fletcher stayed on CBS for 12 seasons with ``Murder, She Wrote.''

(5) Larry Hagman proved that bad guys finish first, playing J.R. Ewing for 14 seasons on CBS' ``Dallas.''