NEW YORK -- If there were ever a time for Michael J. Fox to be less than the nice guy everyone knows him to be, this is it.

He's been in this studio on East 23rd Street all day long, doing those satellite hookups where stars talk into a camera to unseen interviewers. Doing 15 of them, to be exact, at five-minute intervals: "AM Philadelphia," "Sun Up San Diego," "Good Morning Houston." Afterward, the publicists' schedule called for him to be charming to a British reporter and gracious to an Australian one and delightful to a Baltimorean. Now that it's the end of the day, he's entitled to snarl, or at least yawn.

But that would not be nice, and Fox is famous for being nice. So he shakes hands warmly and offers a Diet Pepsi (with Pepsi paying him multimillions over two years to star in commercials, he's not about to push Dr Pepper) and asks the photographer to try to avoid pictures that show a cigarette dangling from his mouth (many of his staunchest fans are children).

And he says that no, he didn't mind at all doing 15 satellite interviews in a row. "It was fine," he assures. "It was okay. I had a good time. For all you know, they're on the other side {of the camera} going" -- here he pantomimes an interviewer retching. Fox is almost as famous for his self-deprecating humor as for his niceness. But he'snot really worried. "I like people," he concludes, smiling, "so generally I do okay."

No argument there.

It's been seven years since the young Canadian dropped out of high school and hit Hollywood, five years since he was cast in what turned out to be such a popular television series (NBC's "Family Ties") that he now has more clout than any small-screen star except Bill Cosby. It's been two years since "Back to the Future," 1985's top-grossing movie, established Fox's box office bankability and guaranteed a ceaseless stream of film offers. At 25, he's wealthy enough to retire.

But despite the notorious combustibility of such elements, Fox has yet to (a) get arrested, (b) check into a detox program, (c) get divorced (or married), (d) adopt a guru (though he does like to hang around with "Steven" -- Spielberg, that is -- eating matzo brei), (e) give a snotty interview or even (f) refuse requests for autographs.

"All the schmaltz you've heard is true," reports Alan Poul, associate producer of Fox's last flick, Paul Schrader's "Light of Day."

"All perfectly true," agrees Herbert Ross, producer-director of the new Fox film, "The Secret of My Success," which opens tomorrow. "He's one of the most sensitive, modest people I know and I'm devoted to him."

"He just is that nice," says Norman Parker, who's played Uncle Rob on several "Family Ties" episodes. "A sweet, sweet person."

"Jeez, it's boring, isn't it?" says Fox, grinning and shaking his head. He doesn't even have a five o'clock shadow.

Wait, did we leave out hard-working? That's the other thing Fox is famous for. In 1985 he'd shoot his series all day, then dash over to a Universal sound stage to shoot "Back to the Future" half the night. In 1986 he squeezed two back-to-back movie roles into the show's five-month hiatus.

"On the one hand, I am a little driven," Fox is saying, cigarette in hand. "On the other, I'm not quite as driven as it appears."

Last Christmas, he relates, he took a week off from work, "took a serious look at my health, calmed down a little ... Humans just can't work this much, especially in an occupation that taxes your emotions this much ..."

Following this exercise in self-analysis, Fox made some changes. He adapted his workout regimen. He Californiafied his diet: "no fish and chips for lunch and a Fatburger for dinner." He quit smoking those Canadian cigarettes ("Marlboros of the North"), for five days anyway ("I was frightened for my dogs"). Next time, he'll quit for good, he vows: "I have no defense; it's just a stupid thing to do."

Andhe had a little talk with himself. "You just can't make everybody happy all the time," he reasoned. "If you're working on a scene where you have to ask the crew to leave for a while, while you prepare, you can't worry that they won't like you. You just say, 'Hey, guys ...' "

Take that! But Fox couches the imaginary request so appealingly, with a shrug and an apologetic smile, that the imaginary crew would surely clap him on the back understandingly and say, "Sure, Mike," then go home and tell the kids what a pussycat that Michael J. Fox is. The boy can't help it.

InCleveland to promote "Light of Day" a couple of months ago, Fox found himself booked into a hotel that was also hosting a dance convention for about 300 preteen girls. "They were wild, camping outside his door; I'd never seen anything like it," says a Tri-Star publicist. "He'd sort of wave, let people in elevators snap photos. He was courteous and friendly to everyone. You would think he'd occasionally say, 'Oh, God, I can't take it.' I never heard it."

Still, he is slowing down a bit. Before shooting on his fifth film begins in Manhattan next week, Fox will have taken five whole weeks off (minus the occasional publicity blitz, of course). "I've always wanted to have some time in New York and just absorb it and get over my fear of it," he says. "Well, fear's probably the wrong word. Awe! I grew up in Vancouver."

There is such a thing as a Michael J. Fox character, someone young and energetic, somewhat bewildered at the situation in which he finds himself, yet plucky enough to take on the challenge. He's honorably loyal to family and friends. He's quick with a joke, but soft of heart. Virtually all Fox's roles, from Alex P. Keaton of "Family Ties" to Marty McFly of "Back to the Future" to the nameless hero of the Diet Pepsi commercial, are variations on this nice-guy-in-a-jam.

Infact, Herbert Ross wanted Fox to play the ambitious Brantley Foster in "The Secret of My Success" partly because the actor's likableness would ameliorate some of the character's questionable exploits (like sleeping with the boss' wife). "It's impossible to perceive him in a negative light," Ross says of Fox. "Another actor in the role might be seen as a hustler; Michael can overcome that and imbue the character with all kinds of good qualities."

Such amiability could eventually prove restrictive, though. Fox could have trouble playing Hamlet, or Hitler, or certain other parts. "It would be difficult for him to play a nihilistic character, or an immoral one," Ross says.

Indeed, when "Light of Day" was pulled from theaters after only a month in general release, the filmmakers partly blamed Fox's different sort of role. Critics had praised his restrained portrayal of a blue-collar rock 'n' roller (Fox "makes it clear that he will be well suited to dramatic roles when the right ones come along," The New York Times said). But though Joe Rasnick was another of those nice guys, he wasn't funny; he spent a lot of time in hospital corridors watching his mother die of cancer.

" 'Lightof Day' was a direct challenge to the expectations of Michael J. Fox fans -- and they wouldn't buy it," theorizes Poul, the associate producer. "We tried very hard in the ad campaign to prepare people {for the fact} that this was not the Michael J. Fox they were used to seeing. But they went in, and when it wasn't light and it wasn't fluff, they turned their backs on it."

"I knew it wasn't 'Back to the Future,' " Fox says. "It was almost me proving something I'd been saying: The pressure's off now; I can do something for its own sake. I would've liked to have seen it do better ... but I'm not in the mourning business. On to the next thing."

No one will accuse the next thing of insufficient fluffiness. "The Secret of My Success," in which Fox hurtles from the mailroom to the executive lounge of a New York conglomerate, bears so little resemblance to reality that Fox has become a bit defensive about it. "It's essentially a fantasy," he insists. "A kid coming from the Outer Village to the Big Kingdom and slaying dragons." It's a little like the Michael J. Fox story, he concedes, "if I stop and analyze it, which I do rarely."

Anyway, the nice guy finally gets a couple of bedroom scenes. "I get to suck face in this movie," Fox says, with that grin. The heroine could have been forgiven if she'd pinched his clean-cut cheek in return.

Whatremains to be seen is how much of a not-so-nice guy Fox can play (and how well) and still have people line up to see his movies. This watershed may come soon: The protagonist of "Bright Lights, Big City," which Fox will start filming next week with director Joyce Chopra, is a club-crawler who consumes large quantities of cocaine.

"It's a very anticocaine book," Fox points out quickly. "The last scene in the book is this guy stumbling down an abandoned street at 6 in the morning with blood flowing from his nose. It doesn't say, 'Hey, go out and buy a gram, folks.' " Still, it may come as a shock to his fans (not to mention the Pepsi people, who consider him "a fabulous role model").

"I take that very seriously," Fox says, very seriously. "On the other hand, I'm an actor. I don't feel any responsibility for the character's actions during the course of the story ...

"Do you really think I sit in a circle with a bunch of people and say, 'What about Michael's image?' I just read scripts and say, 'Jeez, there are some terrific people involved in this and it's a terrific character,' and I play it." And if the part undermines the niceguyness, "I can't worry about that. George Burns is not God, y'know?"

Wait, did we leave out the directing project? That's the other thing in Fox's future, though he fears that given his overstuffed schedule "it'll be ready in the next two centuries."

He's always been a "director groupie" who takes mental notes as he watches rented cassettes of old Frank Capra, John Ford and George Stevens movies. Now he's planning to direct -- well, he can't say what exactly. "I can tell you that it's going to be shot on film, and it'll hopefully be funny."

It all came about in December when the David Letterman show gave each of several celebrities $40,000 to shoot a four-minute film. Fox came up with "The Iceman Hummeth," a bit of whimsy contrasting a hockey player suiting up for a game and a violinist preparing for a concert. The kicker is that it's the orchestra members who start brawling, while the hockey players cluck over the violence. Fox shot it at a skating rink over two nights and -- the truly important accomplishment -- came in under budget. And his pal Steven Spielberg was watching.

"He called me up and said, 'Did you write it?' I said 'Yeah,' " Fox relates. " 'Did you direct it?' 'Yeah.' 'You wanna direct features?' 'Yeah.' And he said, 'If you wanna do it, do it for me.'

"AndI said, 'Nahhh, no way,' " Fox deadpans, a little joke. Because Spielberg's production company is, in fact, backing Michael J. Fox, novice director.

Like he says, he generally does okay.