Treat Williams is the star of two current movies, "Prince of the City" and "The Pursuit of D.B. Cooper," and of "The Pirates of Penzance" on Broadway. He is 30 years old and owns his own plane and made the cover of Newsweek as "A New Breed of Actor."

You might assume he is having a wonderful time. But ah, we know that beneath the surface of success burns the brittle flame of misery and despair and long-suffering art.

"Hi!" Treat Williams said, thrusting out his hand. "I'm hungry! Let's have some dinner!"

In Charlie's, a half-block up Christmasy 45th Street from where his name glows on the marquee of the Minskoff Theater, he orders a hamburger and a beer.

"I cried today," Williams admits revealingly through a corkscrewing grin. "Yeah, I was looking in the window at Macy's, at the Christmas decorations. Lost innocence, y'know? Just remembering those past Christmases when you were a kid and everything was perfect."

In "Prince of the City," Sidney Lumet's gritty tale of a cop who goes straight by ratting on his pals, Williams is clean-shaven, corrupt and tortured by the demons of morality. In real life, he is much the same, except that he has a beard, is on the legitimate stage and seems to be having the time of his life.

In an hour, of course, Williams will once more drag himself back across the street, put on his makeup, do his 100 sit-ups and spring on stage as the Pirate King in Gilbert and Sullivan's "The Pirates of Penzance." True, "Pirates" is the kind of play that, on the surface, infuses both cast and audience with a radiant joy, and has 31 songs and lots of sword-fights.

But this is New York, the embittering, soul-devouring city. Flee, or be destroyed.

"I like New York a lot," Williams said. "It's kind of manic, though, so I have a place in Vermont. I fly up there in my plane with my girlfriend every Monday on my day off. It's five hours by car but only an hour and a half by plane. Things really slow down up there. I love it."

Williams has a Seneca II aircraft, a twin-engine plane with a cruising speed of 210 miles an hour. Apparently, he's not broke. He's had his pilot's license since he was a teen-ager in Rowayton, Conn., and has previously owned a stunt plane.

Asked if he flew upside down and did loops and flew straight down and such when he owned the stunt plane, he replied: "Yes."

He also admitted liking horseback riding, skiing, sailing, hockey, baseball, football (he played football for Franklin and Marshall College) and robust outdoorsy endeavors, some of which he pursued concurrent with his career.

"I got to learn how to ride horses in 'Hair,' " he said. It was his portrayal of Berger the Hippie in that Milos Forman movie that brought him to public attention. Berger was an achetypal good-humored, dancing, singing, wise-in-the-ways-of-the-world character, pretty much in tune with what Williams himself appears at first glance to be.

"I had to ride badly," he explained, "so I had to learn to ride pretty well. I got a real kick out of it. We were shooting in Central Park and every time Milos stopped shooting I'd say, 'How long before the next take?' -- and then be off riding for an hour and a half."

In "The Pursuit of D.B. Cooper," a twangy caper film which opened a few weeks ago, Williams plays the legendary hijacker who bailed out of a 727 with $200,000 in cash, never to be heard from again, so far. In the movie, Cooper is not a psychopath but a lovable goofball who returns to his wife (Kathryn Harrold), stashes the loot and is pursued relentlessly thereafter by Robert Duvall as a bounty-hunting insurance investigator.

"Cooper" is not making anybody rich, but it is full of outdoors stunts and Williams enjoyed making it. "I got to do some white-water rafting, which was great, and a lot of riding, and they let me fly the plane some, too," he said. The plane is an old Stearman crop-duster with an open cockpit used for one death-defying chase episode. "They only let me fly it a little, though," he added.

Curtain time nigh, it was time to trudge back to the evening's work. This he did in a loping stride interrupted by various dance steps, a leaped-over fire hydrant, and what a simpler age used to call high jinks.

After the intensity of "Prince of the City," it is rather a surprise to see him in full comic form, athletic and full of pizazz. But that is just what "Pirates" calls for, a lot of lusty singing, mugging and comic "takes."

He plays the role very broadly, and it was his idea. "They just tell you 'the role can be as big as you want it to be, as long as you can fill it up.' Just don't go halfway -- it's all the way or not at all.

"You really want the house to be packed," he said (it hadn't been the night before). "Comedy is like tennis, and the audience is the other player. You're waiting for that laugh, and then you go on to the next line, and when it's working there's a great rhythm to the performance. Bert Lahr was great at it.

"Actually, if you cut the laughs out of a great comic sequence, it would seem very staccato. The laughs and the lines are all connected. That's why when things are right a piece of business can develop into something else, and that's when it's fun.

"With a movie, of course, you don't have that feedback. The Marx Brothers used to take their movie scripts on the road, and do them before audiences and time the laughs. Then they'd go make the movie and leave just that much time in for the laughs."

Does he consider himself a . . .

"Me, a comic?" Williams says. "Oh God, no. I'm an actor. Comics are unhappy people."

He has been Danny Zuko in Broadway's "Grease," singing and dancing and using a tube and a half of pomade nightly; he had a small role in the film "The Eagle Has Landed"; he starred in a movie called "Why Would I Lie?" that came and went on the same day earlier this year. He played the part of the aggressively amorous soldier in Steven Spielberg's "1941," an unsuccessfully circusy extravaganza. None of his movies since "Hair" has been a big hit, although Williams himself has been noticed. "Prince of the City" was clearly his most demanding role, and he believes it to be his best work.

Actors, of course, must draw on autobiographical conflicts to fulfill their work: pain, anguish, etc. Williams himself is from Connecticut -- as was Eugene O'Neill -- and it was there that he met his first tests.

Williams first had to overcome the fact that Rowayton was a popular summer retreat for Broadway actors, and that his family had connections with the theater. There was always the danger that he would not be signed up by the William Morris Agency until he was out of college. But he overcame all these handicaps -- he was signed by William Morris while at Franklin and Marshall -- and as soon as he graduated he moved to New York City and began working steady.

As if that were not a difficult enough course for a likable young man with a good singing voice, a certain skill at dancing, and a first name taken from the Declaration of Independence (an ancestor is Robert Treat Paine), there were also sailboats.

"When I was a kid, we raced Lightnings three-person dinghies at the yacht club. Boy, I loved that. Sailboat racing takes guts, and real understanding of meteorology, and the brain of a chess player. I have great respect for anyone who does it well. One time I got a ninth in the Long Island Sound Midget Championships. It was only a ninth, but I'm still proud of it."

Despite it all, Williams retained a zany sense of humor. Now, however, it was time for him to go to work, and for the gang in his dressing room to knock off the impromptu rendition of "Winter Wonderland" prompted by the large set of sleighbells Williams had bought some lucky duckling as a Christmas present.

He strips to his briefs and hits the floor.

It was Beverly D'Angelo, on the set of "Hair," who first let out word of his pre-performance regimen of 100 sit-ups. He commences huffing, puffing, sitting up, stretching out and singing scales up ("ma me mo mu ma me mo mu?") and down ("ma me mo mu ma me mo mu!"), interrupting himself only for an anguished shout signifying the fear and tension lurking just beneath his ebullient mien.

"I hate sit-ups but I got to get skinny so then I can wear clothes by Halston!"

He dabs on his makeup, sprays his bushy head of hair into a swashbuckler's mop and waits while his clothier tapes on a Dynex-brand body microphone. A sword and buckler complete the ensemble. Gosh, he seems to be having as much fun as Douglas Fairbanks.

"Yeah, but people really thought he was doing all that stuff. I jump around just as much, but if it works they just think I'm funny."

Inside, he is tortured. Right?

"Ma me mo mu ma me mo mu?" "Ma me mo mu ma me mo mu!"

At five minutes to 8 o'clock he throws everyone out ("Concentration!"), then emerges a moment later to stride backstage. On the other side of the curtains the audience at the Minskoff hushes as the band strikes up its overture. A pirate ship waits in the darkness, and Williams climbs onto its bow, sword held high.

Robby Bensen, who plays the king's pal Frederic, is poised amidships, shining a flashlight in his eyes in preparation for the blinding stagelight. Six other pirates board in the whispery backstage blackness. On cue, they all begin to yell bloodthirstily, the motorized ship begins to roll on its hidden wheels, and into the limelight springs the gang of lovable cutthroats.

Dancing! Singing! Gilbert and Sullivan!

It is a frightful thing, how our artists suffer for their art.