THE PANHANDLERS were comatose in the New York heat, the joggers dissolving in tepid pools of sweat, the shopping-bag ladies begging for the strength to go on, and in Monsignor McGoldrick Park in Brooklyn, wilted leaves were falling from trees and littering Marty Brest's carefully manicured domain.

"They're falling a little early -- it's only 11:30," he said, smiling and looking at his watch as his crew policed the area with huge vacuums. It was a nervous joke, and if Marty Brest felt a bit nervous, he had his reasons.

A wiry, bearded man with the wildly intense look of a renegade yeshiva student, he was directing his first major studio film, "Going in Style," taken from his own screenplay. He is only 28, while his trio of stars -- George Burns, Art Carney and Lee Strasberg -- are old enough to be his grandparents and have two Oscars and a century and a half of show business experience among them.

And if that weren't unsettling enough, consider this. Last year Warner Brothers released two of its most successful films ever -- "Superman" and Clint Eastwood's "Every Which Way But Loose" -- in its prestigious Christmas slot. This year that slot and the estimated 700 theaters that go with it belong exclusively to Marty Brest's Dec. 25 feature debut. He is, in effect, going right from the minor leagues to the World Series, bypassing the apprenticeship on small-scale studio features that almost all of Hollywood's current power elite -- Spielberg, Lucas, Scorsese, among others -- had to endure.

"I've been in the movie business for 17 years, and I'm not aware of anything like this happening before," says co-producer Tony Bill from his Venice, Calif., office. Fred Gallo, Bill's opposite number in New York, calls it "a fairy tale. All those kids in film school must be seeing this and going crazy."

Obviously, Marty Brest is rather special. Though general audiences have never heard his name, critics and industry types alike consider him perhaps the most talented director under 30. His 73-minute student film, "Hot Tomorrows," made under the aegis of the American Film Institute and a sensation at both the New York Film Festival and Filmex, was so impressive even in the rough cut that Warner's, a studio not celebrated for its soft heart, advanced Brest a no-strings-attached $15,000 to finish it. (It has, however, never been released commercially.)

Shot almost entirely on location in New York, "Going in Style" centers on three retirees -- Burns, Carney and Strasberg -- who pool their $237 monthly social security checks in order to share an apartment in Astoria. "They haven't missed anything in life -- they're just old, people are passing them by and they're waiting to die," explains Strasberg. Bored with the wait and anxious to reaffirm their vitality, the trio plans and executes a bank robbery that alters their lives more than they expect.

But "Going in Style" is in no sense a caper movie. It is instead almost Chekhovian, an affectionate character study that deals with the theme that-formed the crux of "Hot Tomorrows": old age.

"I seem to have this fascination with the aging process," the director says at an outdoor cafe a few blocks from the luxury Manhattan apartment building where he's been incongruously placed during the shooting. "I think about it a lot, how the essence of a drooling old man in a hospital somewhere who doesn't remember his name is the same as when he was a healthy, virile 18-year-old kid. People are put in all these different forms, but they're the same all the time. There is a continuity of souls no matter how they're housed."

Curiously enough, for all his nascent preeminence, Brest is not one of those people who's always been burning to enter the movie business. "I'm not a film buff, not at all," he says cheerfully. "I never really liked to watch movies when I was a kid, and I don't like them that much now. It's not quite healthy for me. I could use more education. Without it, I have to struggle a lot more, I have to come up with stuff on my own that other directors have already figured out."

Brest is a graduate of New York University's School of Film, but he says he went there just out of a general interest in production, in the idea of making something. He soon realized that "the director is the person who tells everyone else what to do," and decided that was for him. In his second year at NYU he made what he calls "my $800 opus," an award-winning short called "Hot Dogs for Gauguin" starring Danny De Vito, now of TV's "Taxi." It was good enough to get him into the AFI's Center for Advanced Film Studies in Beverly Hills, where "Hot Tomorrows" came into his life.

While Brest was sneaking an overlength, over-budget "Hot Tomorrows" past the AFI, a sometime-producer, sometime-actor named Leonard Gaines came to Brest with a half-hour tape in which the basic plot outline of what was to be "Going in Style" was laid out. The tape had been made by one Edward ycannon, a carpenter from Queens."The man was a great storyteller," Brest says, "and as soon as I heard it I knew it was perfect for me." After securing the rights, he took the project to Warner's and, in the first step of the fairy tale, got the go-ahead to direct the film.

At first Warner's was reluctant to let Brest write the screenplay himself, but after they failed to find a suitable writer, he talked them into it and they liked the result. He then turned his attention to casting. "I originally had unknowns in mind for the old men, but I discovered that if you're not known by that age, there's usually a reason for it," he says, older now and wiser. Then, too, Warner's having agreed to put $5.5 million into the film, had expressed an understandable yearning for some familiar names.

First cast was Art Carney, whom Brest admits he had in mind for the part of Al, a restired bartender, when the script was little more than half finished. For Willie, a retired cab driver with a bit of an artistic sensibility, Brest had leaned toward Barnard Hughes, star of the Broadway show "Da," but NBC had an option on a TV pilot Hughes had done and his availability for the entire shooting was questionable. So the role went instead to Lee Strasberg, artistic director of the Actors Studio for 30 years but an actor himself only since "Godfather II."

The casting of Burns as Joe, a retired clothing salesman who is the ringleader of the trio, now seems like a brilliant idea, but it was an idea that Brest, who had wanted Burgess Meredith, resisted for quite some time.

"When I first heard his name I had the idea he wasn't what I had in mind," the director admits. "I saw George Burns as a very savvy, show business kind of figure. It didn't mesh with the idea of a schlumpy old guy on social security." Finally Brest was prevailed upon to at least meet the man, and after that it was no contest. "You can see the wheels going in his head," he says, pretty much in awe, whirling his hand for emphasis. "I think George Burns in just about the hippest person on the face of the earth."

While "Going in Style" was still in preproduction, word filtered out of London that reclusive director Stanley Kubrick would not have his "The Shining," which will star Jack Nicholson, ready in time to be Warner's Christmas release as scheduled. "What happened was totally in tune with the other Alice in Wonderland aspects of the project," says Brest, who still can't quite believe it. "Warner's said, 'Kubrick's picture's not going to be ready. Can we use yours?' I mean, what the hell? That concept sent me right through the wall. I felt like dropping him a note saying, 'Dear Stan, take it easy, I've got everything covered.'

"It's like inheriting a million dollars and putting it all on red 34. The first day, I was almost fainting with terror. Even now when I'm on the set it's like I'm being hunted -- all my senses are alert. It's my life at stake."

Brest's New York coproducer, Fred Gallo, is a burly man who drinks coffee out of a mug decorated with a caricature of himself scowling and saying We're Over Budget. Gallo is one of the paramount nuts-and-bolts men in the business, a veteran of 35 films and winner of Director Guild awards for his work on "The Godfather," "Rocky" and "Annie Hall." When he says, "I like to be on top of things," he is not just making conversation.

"I hand-picked this crew. I got the best people I could," Gallo says resolutely. "I got the sound man who works with Woody Allen because Woody doesn't like to loop [re-record the sound]. I fought with the unions to get cinematographer Billy Williams ["Women In Love," "Sunday, Bloody Sunday"] in from England. The gaffer, the grip -- are all guys I've known since they were fifth and sixth gaffers and grips. The idea was even if Marty fell on his face, we could still put something together for Christmas."

The director did not falter, but something totally unexpected did take place. Art Carney, walking back to his New York hotel after a steak dinner, was accosted by a man who said he was a fan and asked for money. Carney offered a dollar, the man said it wasn't enough, and as the actor searched his pockets for more the man smashed him in the eye with his fist. "If you sent a guy to makeup and asked for a black eye and this came back," Gallo says, "you'd say, 'Don't be ridiculous, no eye ever looked that bad.'"

Carney eye injury was serious enough to halt production for two full weeks. This pushed Brest into a savage production schedule. His average working day, which began at 4:30 a.m. and ended at 9 or 10 at night, including evenings and Saturdays devoted to editing, got even more hectic during the very abbreviated four to five weeks that were allocated for postproduction work.

"I don't know where he gets his energy," says Gallo, weary just thinking about it. "The first two weeks of shooting we had a lot of weather problems. Even though it was an 8 a.m. call, at 5:30 he'd be on the phone saying, 'It's cloudy, what're we going to do?' I'd say, 'Marty, it's only 5:30, it'll clear up.'

"And when he'd arrive on the set it was boom, like he was shot out of a cannon. This crew of 40 to 50 people has had a thousand years of experience between them and their enthusiasm is not like his. That grip has put up 100,000 miles of dolly tracks. When he arrives in the morning he's not leaping out of the car the way Marty is."

To see Marty Brest on the set in McGoldrick Park in Brooklyn is to understand what Fred Gallo means. Loose and limber, rocking on his heels, snapping his fingers, he is constantly in motion, now rushing forward to pat a rebellious hair into place on George Burns's scalp, then joking with a crew member about a shot he's setting up: "It's a movie trick, I read about it once in a book." He looks very much at ease.

Still, relations with the crew were often precarious. "The biggest difference from 'Hot Tomorrows' was that you can't work 24 hours in a stretch. I found things like specific lunch hours very frustrating," Brest says, more or less seriously."The crew rolls their eyes like 'Here comes this schmuck kid.' I used a view finder one day and Fred Gallo kind of sidled up to me and said, 'Please put that thing away, or we're not going to have a crew left.'"

Before "Going in Style" went into production, Brest went to a few directors he knew and asked, "What do you wish somebody had told you before you made your first feature -- just something simple that would've saved your ass?' And Marty Scorsese gave me some really good advice: Don't go out. Just stay at home, eat and go to sleep. Keep all your energy for when you're on the set."

Now that "Going in Style" is finished, all Marty Brest wants to do is direct another feature and, if possible, be more fanatical about the task than he's been already.

"I'm not really as meticulous as I should be," he says, smiling. "When I have more experience, when I get to be more at one with the task, I'm going to be worse. People are already telling me, 'If this film makes it, you're going to be impossible to deal with,' and they're right. Then I won't allow myself to be compromised on any point. Then," and here the smile really breaks open, "then it'll be Kubrick who finished shooting way before Marty Brest."