Key to series longevity? It's the writing, stupid

The key to shaking up a cast is a really fresh face.

“You have to make sure that when you switch a regular you come up with a different character — this is not a situation where ‘Tonight Hamlet will be played by ....”’ Wolf said.

S. Epatha Merkerson and her police lieutenant on “Law & Order” are “vastly different” than her predecessor, Dann Florek and the part he played, Wolf said. (Florek now runs the precinct on “Law & Order: Special Victims Unit.”)

Sometimes a cast change is so right it warrants a whole different direction — another series.

Entering its eighth year, Kelley’s “The Practice” had run out of steam and into budget cuts. Kelley decided on a cast shakeup for what was expected to be its final season.

Exit Dylan McDermott and a number of other stars and enter James Spader as slick, unconventional attorney Alan Shore, a sharp contrast to the original hardscrabble crew on “The Practice.”

“The liberating thing for me was, OK, we can bring in a character that does not have to be redeeming in perpetuity,” Kelley said. “It can be someone who just burns out after a year. ... He could either die or get disbarred and I can just let him walk the plank.”

The creative license had the effect of boosting the show’s ratings enough to earn ABC’s nod for renewal. But Kelley, facing what he called a tough decision, decided “The Practice” had turned in Shore’s direction, and so had the future: “Boston Legal” (10 p.m. EST Tuesday), starring Spader and William Shatner, debuted in 2004.

But a series that’s started to founder can be saved. It happened with “ER,” said executive producer Zabel, who’s worked on the show six years.

“I think there was a period when we lost the balance of comedy, drama, the quirkiness, the sort of social realism of the show, the romance,” Zabel said. “What we’ve done is rediscover the chemistry that was there in the early years of the show.”

Audiences seem to be responding. Despite lead-in shows struggling in a crowded field, “ER” is dominant among advertiser-favored young adults at 10 p.m. EST Thursday and is competitive in total viewers.

In season No. 13, an average 13.7 million viewers weekly is nothing to sneeze at.

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