`SEINFELD' STAR STILLER REFLECTS ON END OF ERA.

Byline: Keith Marder Daily News Television Writer

When Jerry Stiller walks down a street in Manhattan, perfect strangers approach him and chant: ``You are a very sick man!''

Other times, he's greeted with the call: ``Serenity Now!''

And believe it or not, those are just two of the perks of his job.

Stiller, an entertainer for about half a century, is probably best known to a couple of generations as Frank Costanza, father of George on ``Seinfeld.'' It was the elder Costanza who first shouted in his recognizable staccato those memorable lines about sick men and serenity.

``For a while, I didn't know what I was getting into,'' said Stiller, who plays the easily irritated, high-strung, catch-phrase-spewing nut on the popular Thursday night NBC sitcom. ``But the show grew, and grew and suddenly I was popular again.''

Stiller said that he has yet to absorb the news that will likely be the death of Frank Costanza - that ``Seinfeld'' will cease production after this, its ninth season.

Series star and creator Jerry Seinfeld announced last week that the most successful show of the past decade will make this season its last.

``It's rather sad that this is happening,'' said Stiller, who will appear in at least three more episodes. ``I hope this generation of viewers sees this show the same way I look at `The Honeymooners' and `The Jackie Gleason Show.' They don't go away.''

Neither has Stiller. Decades after many appearances on ``The Ed Sullivan Show'' with his wife and comedy partner Anne Meara, there is probably more heat on Stiller than ever before.

``Since I have been on `Seinfeld,' I have done four plays, three on Broadway, and had a hip replacement,'' said Stiller, 70. ``I have been able to do everything I have wanted to do and crammed them into the last year of life all because of `Seinfeld.' ''

The exposure has also led to a speech while cruising on the Queen Elizabeth II ocean liner, stardom in Germany and Australia and a TV Guide cover - in New Zealand.

``That's when you really know you're famous,'' he said, ``when they want you in New Zealand.''

Imagine if he appeared on the show more than a few dozen times. Stiller is fast to credit Seinfeld and his unselfishness, which has led to the quality of the show.

``Everyone talks about how this guy Jerry Seinfeld has made a bundle,'' said Stiller, who has earned an Emmy nomination for his work. ``But he has been able to give up the ball to other actors. The show can be spun off for any one of the three other people involved. He has certainly let them go wild, so to speak, which has been very helpful to the story lines.''

As Frank Costanza, Stiller has certainly been given latitude to emote and steal every scene he is in. Stiller considers ``Seinfeld'' as inventive for its time as many top comics through time. He puts the show in the same league with Abbott and Costello, Gleason and Carney and the Marx Brothers.

``They brought four stories together,'' Stiller said of the typical ``Seinfeld'' episode. ``They brought back characters from two years earlier and revitalized the story. That has never been done before, and people found it innovative, and they liked it. Some part of me said that maybe he shouldn't have stopped because he had us exploring new waters that have never been tested - his own psychological thinking.''

As much as he has enjoyed the run, it's time for Stiller to move on. When talk turns to spinoffs, Frank Costanza's name often comes up.

``I wish it would have happened 10 years ago. Right now I'm entertaining anything that comes along that gives me a chance to do something as a continuation of Jerry Stiller, not Frank Costanza,'' Stiller said. ``I, like the other people on the show, am having to fend for myself. I want to continue some kind of work so I can say I'm an actor.''

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PHOTO Jerry Stiller, who plays George Costanza's father on ``Seinfeld,'' credits his recurring role on the sitcom for his suddenly busy career.