NEW YORK — Sam Waterston said he subscribes to Meryl Streep's advice: "Stand up for your character."
So after Fred Thompson left NBC's "Law & Order" in June to pursue a presidential bid, Waterston initially rejected the notion of Jack McCoy, the acerbic prosecutor Waterston has played on the show for the last 13 seasons, succeeding Thompson's Arthur Branch as district attorney.
"Jack McCoy has been sort of anti-politics all his life and loves his job, so I couldn't think of any reason why he'd want to," the actor said. "Then they started talking to me about it, and it began to sound really interesting. Here you would have a guy insulting politics all his life forced into a political role. And you wouldn't have to do any exposition, because everyone would know it."
When "Law & Order" returns for its 18th season Wednesday, viewers will find McCoy already ensconced in his new office. The change has meant a decidedly new dynamic for the drama, whose future seemed precarious last season after its ratings dropped 19%.
"I think it's the most important paradigm shift since the fourth season, when women came into the show," said series creator Dick Wolf. "It's a really interesting turn. He is dealing with what a lot of guys our age deal with -- generational transfer. He's really a lion in winter."
The series' new focus comes as it makes its scheduled midseason return, but with an extra advantage as a result of the ongoing writers strike: at least 12 new episodes.
"We're going to be up against a lot of repeats, so I'm hopeful that the message gets out that we're all originals," Wolf said.
This season finds McCoy, after years of bucking the bureaucracy, contending with a new foil: Michael Cutter, an aggressive young prosecutor played by British actor Linus Roache. The chief assistant district attorney is cast from a different mold than his boss -- he wears dark dress shirts and does legal research on his BlackBerry -- but they share some traits.
"He's a bit of an attack dog, willing to go that extra mile to bring justice to bear," said Roache, who was most recently seen on NBC's "Kidnapped." "We've had some episodes that have been really fun, where there's almost been a Jack Bauer [the renegade agent from Fox's "24"] approach to the law."
"Law & Order" viewers are accustomed to seeing new characters cycle through the long-running crime procedural. (With the arrival of Roache and Jeremy Sisto, who joins the show this year as Det. Cyrus Lupo, the series has now had two dozen actors play one of the six main roles, Wolf noted.)
Still, this season's shift is a dramatic one: For the first time, McCoy won't be tussling with judges and pounding witnesses in the courtroom. Instead, he'll weigh in on cases from his new perch, a role that has been expanded to give Waterston more of a presence than his predecessors.
"You will still see the same level of intellectual and moral combat going on," Wolf said. "He and Linus are actually very different types of prosecutors, so there's a lot of real conflict over methodology."
Waterston acknowledged that viewers may initially find his new position jarring.
"The normal human response to change is, ooh, is it going to be OK?" he said. "I think the changes have brought a whole lot of fresh life to the show, so once people get over their initial worry that change might not be nice, I think they'll like it."
For his part, the 67-year-old actor appears to be enjoying the setup. As he passed Roache on his way to the set on a recent afternoon, Waterston grabbed him in a loose headlock and playfully pulled him along.
"Working with Sam is just a total joy," said Roach, 43. "We just share a lot of mutual passions about where we are at this time as a human species and what matters. You know how you just meet a human being and there's a connection? It's just there."
It helps that they get along, because following McCoy "is a daunting thing," he added. "There's a moral function that that role has, so there's quite a bit riding on it."
With Cutter, "the energy may be different," Roache said. "But the essence of the show will still be there: How do you deal with these incredibly complex, challenging, human predicaments?"
Waterston said he misses doing the courtroom scenes a bit, but pronounced himself delighted with the series this season.
"Part of it is that Linus is so good," he said. "You don't have to feel like you've left a hole or anything like that, because it's all filled up and then some."
Waterston, a classically trained actor who frequently performs in New York-area theater productions when he's on hiatus, said he never envisioned that he would be on "Law & Order" this long.
"Not for a minute!" he said during an interview in his unadorned dressing room at Manhattan's Chelsea Piers, where the series is shot.