Sam Waterston’s long march into television history continues Wednesday night.

Waterson has played the crusading Jack McCoy in 327 episodes of the NBC crime drama “Law & Order.” It becomes 328 Wednesday when the 19th season opens.

Only a handful of actors have played the same role on a prime-time scripted series longer than the “Law & Order” star. That includes Waterston’s co-star S. Epatha Merkerson, who portrays Lt. Anita Van Buren, with 350 episodes.

The all-time champ is James Arness, who walked the streets of Dodge City as Marshal Matt Dillon on 616 episodes of “Gunsmoke.” Others ahead of Waterston include: “Bonanza” stars Lorne Greene (415), Michael Landon (398) and Dan Blocker (394); “Knots Landing” stars Michelle Lee (344) and Ted Shakelford (333); and “Dallas” star Larry Hagman (357 episodes). Just for the record, Kelsey Grammer played Frasier Crane in a total of 464 episodes if you count both “Frasier” and “Cheers.”

Waterston never dreamed back in 1994 when he signed on to play the role that he would be reaching such a longevity record in 2008.

“I had no idea how long the show was going to last. But I knew I would only be with it for a year,” Waterston says during a telephone conference call with television critics.

Waterston, 68, has signed a one-year deal each season. The actor says series creator Dick Wolf made a clever creative move by giving Waterston one-year deals each season.

“It’s part of the reason why I’m here so long,” Waterston says. “They have made a long leash and allowed me to sign up year after year after year. I never felt imprisoned here. I’ve always felt free to stay. Free to go.”

That freedom has allowed Waterston to take chances and shape the character so the audience would find him interesting. But that doesn’t mean there are major personality changes coming to Jack McCoy.

“The character has been long established. The main thing is not developing him but keeping him consistent with himself. I’d say that’s partly my job and partly the writers’ job,” Waterston says.

The Emmy and Golden Globe winner’s role has changed slightly in recent years. Linus Roache took over courtroom duties with his role as Michael Cutter, executive assistant district attorney. Waterston took over the job as district attorney.

The career change for Waterston’s character did not make the actor question his place with the show. Waterston thinks “Law & Order” has the best writing, acting and direction in television. And because of the quality he sees and experiences, he thinks he has the best job in TV and wants to stay as long as Wolf and the network want him.

Thanks to reruns, it amuses him how viewers have been able to see him age over the years.

He laughs and says, “I look back and I think why didn’t I enjoy myself more? I don’t look any better today than I did then. I should’ve just counted myself lucky to be as presentable as I was.”

‘Law & Order’

10 p.m.Wednesday

NBC