Surviving Cast Changes : In TV ensemble series, actors come and go and replacing them is a big challenge; the talents of producers and writers seem to be the glue that holds shows together.

If you can’t name a single current star on NBC’s “Law & Order” who was in the show’s first episode, not to worry: There aren’t any.

Yet that series is hardly the only prime-time drama to experience its own share of drama when it comes to cast changes. ABC renewed “Murder One” for next season without knowing who the leading man would be, while CBS’ “Chicago Hope” is undergoing its third round of casting surgery in as many years.

Many of these shows nevertheless continue to thrive, suggesting that the storytelling qualities provided by their pedigreed producers--including “NYPD Blue” and “Murder One’s” Steven Bochco, “Law & Order’s” Dick Wolf and “Chicago Hope” creator David E. Kelley--may be more important than individual performers, allowing series to roll along despite on-screen turnover.

“As it always is, it’s the writing that supports these characters and presents them in the right way,” said Wolf, whom “Homicide: Life on the Street” executive producer Tom Fontana only half-jokingly called “a hero among writer-producers” based on his ability to keep plugging in new cast members without breaking stride.

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Industry executives say that dynamic makes ensemble dramas more attractive to networks and studios, who can’t be held up as easily by a discontented star. Although they’d prefer not to risk making changes--which, producers note, have in most instances been done out of necessity rather than design--the ability to insert new performers into a show does offer financial benefits as series age and star salaries increase.

Ensemble dramas have always experienced some turnover, as witnessed by “St. Elsewhere” and “L.A. Law.” Yet such defections have seemed more common in the last two years. “Of late it’s been pandemic,” said John Tinker, executive producer of “Chicago Hope.”

Part of that can be attributed to the allure of jumping from television to feature films, as well as the sense among many producers that their shows are bigger than any single actor. As “Homicide’s” Fontana put it: “I think this show now lives without any particular character being the focus.”

Meanwhile, audiences accept casting changes, producers suggest, because of the heightened sense of reality in these programs and an understanding that, in real life, people do move around.

“When you look back five, six, seven years, how many [of the same] people are you still working with?” Wolf pointed out.

“This is like a real place: People come and go,” added Tinker, who is still wrestling with the configuration of next year’s “Chicago Hope” cast. Negotiations continue with fellow “St. Elsewhere” alumnus Mark Harmon, while Ron Silver is in talks regarding at least a limited number of episodes. Roxanne Hart will leave, and it’s undetermined whether Mandy Patinkin will make any more appearances.

Producers themselves remain split whether the trend is to be embraced.

“It would seem, based on my recent past, that I’m a proponent of that, but in truth I’m really not,” said Bochco, who weathered David Caruso’s much-publicized exit from “NYPD Blue” early in its second season. “It’s not my impulse, and I don’t believe it works most of the time. . . . I really think you take a great risk when you do something like that.”

On the other hand, “Law & Order”--which enters its seventh season still looking for someone to replace Jill Hennessy’s prosecutor Claire Kincaid--has seen its ratings improve despite turning over some characters several times. The only survivor from the first season is Steven Hill, who plays the crusty district attorney and started in the second episode.

Wolf thinks adding new faces has helped to keep the series fresh. The new characters, however, can’t be clones of the ones they replace, he noted, but must establish themselves individually.

Fontana agreed, having gone through several departures early in “Homicide’s” run. “I think what it does is great for the credibility of the show, because it really allows you to explore that these people have life-and-death jobs, and that some of them die, some of them crack,” he said.

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Bochco, however, thinks there are core performers even an ensemble show can’t replace. He cites actor Michael Conrad, who played “Hill Street Blues’ ” Sgt. Esterhaus and died during the series. When that happened, Bochco said, “something went out of that show that it never recovered.”

Bochco added that “everybody was nervous as hell” about Caruso leaving “NYPD Blue” to pursue a feature career but that his ace-in-the-hole was co-star Dennis Franz. Though the police drama has lost supporting players (including Sherry Stringfield and Gail O’Grady, who recently left to star in a sitcom pilot), Bochco called Franz and Caruso’s replacement, Jimmy Smits, “untouchables.”

In “Murder One,” Anthony La Paglia will replace Daniel Benzali, whose departure will be addressed. Grace Phillips, one of the attorneys, will also leave, but other cast members in the law firm return.

Bochco has received some negative mail about Benzali’s exit, as he did with Caruso. “Viewers don’t like to feel manipulated, and I understand that,” he said. Tinker added that the audience may be reluctant to invest in characters “if you change too many people, too fast, too often.”

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Still, for most producers, dealing with disgruntled stars falls into the “life’s too short” category.

“If someone comes to me and says, ‘I’m unhappy,’ [and] I can’t fix that, who wants them here?” Tinker said.

Despite a slew of new sitcoms starring Bill Cosby, Ted Danson, Michael J. Fox, Brooke Shields and Rhea Perlman, there’s also a perception that TV often does a better job creating stars than building vehicles around them. Recent examples include the casts on “ER” and “Friends,” as well as once-little-known comics like Roseanne, Tim Allen and Brett Butler.

Television is “the fastest known way to be a movie star these days,” said Joan Hyler, a manager whose clients include “NYPD Blue’s” Sharon Lawrence, who has parlayed her supporting role into starring in an upcoming NBC sitcom while she’s gradually phased out of her current series.

With TV stars increasingly doing movies, ensemble casts provide greater flexibility in working around outside projects.

“If somebody got a movie that they really want to do, the joy of being in an ensemble is you can let them out,” Fontana said. “If you’re the single lead in a show, you can’t say, ‘I need a couple of weeks off.’ ”

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In general, producers say they’d rather actors honor the multiple-year contracts they sign when beginning a new series. From a bottom-line standpoint, though, there can be advantages to being able to substitute in new actors.

The longer a series stays on the air, Wolf noted, the more it costs to produce, and these price hikes--largely attributable to “exponential salary increases”--have “done in a lot of shows over the years.”

Performers and their agents must recognize the difficult economics of one-hour series, he added, and know that producers have options. “There are an awful lot of good actors out there,” he said.