Shortly before the release of 1989's "Casualties of War," Michael J. Fox was feeling pretty proud of himself. After seven years of playing Alex P. Keaton on the hit NBC television series "Family Ties" - a role that basically required him to look cute and regurgitate a series of pluckish one-liners - here he was starring in a Brian De Palma film in an adult role, that of a tortured Vietnam soldier. The media, he says, did not share his enthusiasm.

"When I was doing publicity for that film, I kept getting these questions from reporters like, `What are you trying to prove?"' says Fox, who admits that he is perceived as the perfect boy-next-door. "I kept saying, `There is no agenda with my career, there is no angst.' But I kept getting the perception that people thought I was this tortured little Napoleon Bonaparte figure with something to prove."Fox didn't get mad. Oh, no. He got even.

"The Hard Way," the new action adventure/buddy comedy that pairs him with James Woods, is his just revenge. In the film, Fox has a field day playing a character that is a dead ringer for what he believes is the media's perception of him.

He plays Nick Lang, a spoiled baby-faced star who has had great success in a number of action comedies (films not unlike Fox's three successful "Back to the Future" movies). But box-office success is not enough for Lang: He wants to be taken seriously. And he wants a juicy part in a planned gritty Hollywood movie about cops that he feels will help him accomplish this goal.

Lang's hunger for the role of the Serious Police Officer is so great that before a planned screen test for the part, he flies to New York and hangs with John Moss (Woods), a Dirty Harry-type cop on whom Lang wants to base his performance. The catch: Moss hates Lang. Despises him. And yet, on orders from his star-struck boss, Moss is forced to baby-sit Lang while he tracks down a lunatic serial killer. Their ensuing love-hate relationship is the stuff of "The Hard Way," and, Fox says, of a great screen spoof of unfounded Hollywood stereotypes.

"Jimmy had the same point to make in this film about his work," Fox says. "He's perceived as the world's most intense man, and in this film he gets to make fun of that image. And I got the chance to come in and make fun of my image, which is of a shallow, light-comic movie star who wants to do Hamlet."

Tackling the Bard is a goal that Fox admits he isn't ready for. For one, he's too young. Even though he turns 30 in June, Fox retains the youthful glow of a teenager.

He bounds into an interview wearing oversize Nikes and Levi's, his shaggy feathered mane falling down in his eyes, that familiar nice-guy smile strewed across his face. After shaking off some charm ("Hey! Howsit goin'?"), he then proceeds to open up a can of Diet Pepsi, the soft drink he regularly peddles on all of those overproduced television commercials. "I really do drink the stuff," he says sheepishly, and then takes a great big gulp.

In the flesh, Fox is honest, sweet, funny and youthful - very youthful. A lot like Alex P. Keaton. This, he says, has proven to be his biggest problem.

Hollywood, he says, is happy with the way he is, thank you very much, and doesn't want him to grow up.

"When the whole teeny-bopper thing happened, I kept my head down," he says, referring to that flush of stardom that crept up on him soon after the premiere of "Family Ties" in 1982. "I wanted it to disappear. I was never really comfortable with that. My hope, you know, was that it would gradually melt away. My hope was that when I would grow up, my audience would grow with me. I didn't want people to carry around lunch boxes with my face on them forever."

Fox, to his credit, tried to involve himself in projects that proved he was capable of playing mature characters. After the ratings and box-office success of "Family Ties," "Back to the Future" and "Teen Wolf" - giggly light-hearted projects one and all - Fox starred in the films "Bright Lights, Big City" (in which he played a confused coke fiend), "Light of Day" (in which he was a confused rock musician) and "Casualties of War" (in which he was a confused soldier). Despite his star power, no one showed up to watch his dramatic efforts. The films in which Fox played it tough were all bombs.

But Fox is not discouraged by this. He realizes, he says, that old images die hard. And so, he plans to keep making the lighter-than-air movies that audiences clearly want to see him in and every once in a while do a film like "The Hard Way" to test his mettle.

"The Hard Way" presents Fox in an entirely new light. He is mad, bad and dangerous to know. He's worlds removed from Alex P. Keaton and "Back to the Future's" Marty McFly.

Nick Lang is indeed a juicy creation, the male equivalent of Joan Crawford. A nasty prima donna, he throws temper tantrums, skewers Steven Spielberg for "not mentioning me during his AFI tribute!" and hurls his People's Choice award through a TV screen when his agent tells him he has to play only good guys.

"There was no research involved in this," Fox jokes. "This was cinema verite. I am a monster!"

With "The Hard Way" now in theaters, Fox has turned his attention toward other roles. He is, he jokes, in stretch mode. In "Doc Hollywood," which is being filmed, the actor plays a Beverly Hills plastic surgeon. And he has just finished directing an episode of the series "Tales From the Crypt." Movie star, actor, director - Fox wants it all.

"I'm over my head," he moans. "It's a pure vanity thing, directing. I did a short for the Letterman show and loved it (a film about ice hockey) and wanted to try my hand at it again. A lot of big studios will let me direct, but I wanted to do something small first so I could see if I wasn't a nightmare to work with."

Fox a nightmare? Never. He admits this is true and adds that being perceived as the entertainment industry's Mr. Nice Guy isn't so bad after all.

"If I see a kid with a bike and walk up to him and he looks up and sees my face and I say, `That's a cool bike,' well, I've made that kid's day," Fox says. "What a reward. I mean, plumbers don't get that . . . "