In the long run, it seldom hurts to do decent work. Consider the case of Robert Mandel, belatedly making a name for himself as the director of "F/X," an uncommonly proficient and savory thriller.

A former assistant to Joseph Papp, Mandel shelved a promising Broadway directing career to attend the American Film Institute's Center for Advanced Film Studies in Los Angeles. He completed the two-year course by directing a dramatic short, which led more or less promptly to an "after-school" TV special called "Sunshine's on the Way" and a first feature film -- "Independence Day."

The latter, shot largely on location in 1982 in the small Texas town of Anson, was a memorable little human-interest drama. Those who saw it tended to be most impressed by the confidence and clarity of Mandel's direction and by the disarming brilliance of Dianne Wiest's performance in a supporting role.

The hitch was that almost no one saw it. When a handful of less than ecstatic "test engagements" confirmed the doubts of the Warner Bros. marketing division, the picture was allowed to vanish down one of the ever-yawning gaps in the Hollywood distribution system.

What spared "Independence Day" prolonged obscurity was the emergence of home video. You won't find it all over the shelves, of course, like "Gremlins" or "Porky's Revenge," but now that Dianne Wiest is enjoying wider exposure as Woody Allen's latest romantic foil in "Hannah and Her Sisters," video stores should have the incentive to add a few more copies.

The video factor also came to the professional rescue of Robert Mandel, as Mandel recalled during a recent visit to Washington:

"The producers of 'F/X' , Dodi Fayed and Jack Wiener . . . checked me out by renting a copy of 'Independence Day' at a neighborhood video store. I still wasn't certain why they were interested in me, given the pretext of 'F/X.' I felt no particular desire to direct a thriller, and I didn't think 'Independence Day' revealed some hidden aptitude for the genre."

Well, did these savvy producers ever specify what they saw?

"They said they were looking for an actors' director," Mandel said. "Looking back, I think they can be congratulated for taking an intelligent kind of risk. During the interview they probably read something reassuring in my personality. Although I obviously had no experience directing the kind of thrill-and-action sequences the picture would require, I think they sensed that I'd prepare myself. There's really no alternative when you're doing something like a complicated car chase around the streets of New York."

One of the things that sets "F/X" apart from the run of contemporary thrillers is its surprisingly human scale. The protagonist, a movie special-effects designer played by the Australian actor Bryan Brown, is drawn into a murder conspiracy when he agrees to help fake the apparent public shooting of a government witness.

Up to the finale, when the hero's specialized skills give him an almost superhuman advantage over well-armed adversaries, the movie engages your interest and respect by depicting Brown as a vulnerable figure. It's such a shock to see a hero without martial arts prowess that you realize how much of a cliche' that form of prowess has become.

Mandel's human-interest skills tend to humanize a format that has been pandering far too often to an appetite for gratuitous violence. Indeed, it was fascinating to observe the dynamics between "F/X" and a rowdy, thrill-seeking midnight movie audience. At the beginning, the customers were demanding more sensations than the movie was calculated to deliver. Nevertheless, the film began to impose its own terms on the crowd, and by the time the elaborate action scenes erupted, Mandel seemed to have the audience in the palm of his hand.

The slim and soft-spoken Mandel regards his finesse strictly as a matter of study and application. "I didn't know a thing about the technical side of filmmaking when I went out to the film institute," he says. "I never had the slightest mechanical aptitude either. I was pre-med at Bucknell before getting interested in drama and transferring to Columbia, where Joe Papp was one of the instructors. My whole outlook was theatrical when I arrived in film school, and I thought I might be sunk by my technical ignorance. I just cured the ignorance by studying.

"On the stage you can evoke just about any setting imaginable with a minimum of symbolic material or mere suggestion. Intuitively, you allow the audience to do the rest for you . . . With film you're obliged to evoke settings much more realistically -- to oversimplify, it's better to have a house that looks like a house rather than the mere suggestion of a house. But you can also draw on the evocative power of authentic settings and locations, and you can really control what the audience is seeing."

In three weeks of release, "F/X" (budgeted at $10.5 million) has grossed a very respectable $15 million. According to Mandel, the project began as "a $2 million exploitation vehicle" and underwent progressive upgrading.

The original screen writers, Robert T. Megginson and Gregory Fleeman -- respectively a documentary filmmaker and an actor -- were without previous writing experience. Mandel supervised "three major rewrites to bring the characters into believable working worlds," and a pair of uncredited screen writers, Tom Pope and Alan Ormsby, contributed substantially to the revisions. Ormsby is best known as the writer of "My Bodyguard," but has also worked on a number of horror thrillers. One of them was titled "I Dismember Mama," mentioned in passing in "F/X" as one of the hero's grislier credits.

"For the first time in my life, I'm supposedly responsible for something that's critic-proof," Mandel says. "You can't imagine how odd that term sounds unless you come from the theater, where nothing's critic-proof. Especially in New York, where one critic can pretty much make or break a play. Now my agent assures me I don't have to worry about the reviews of 'F/X,' because it's a so-called 'high concept' entertainment -- the audience can recognize at a glance what they'll be getting.

"Maybe there's something to the recognition element, at least in attracting business from the beginning, which seems to be essential to box-office success. But what's really interesting about movie reviews is this incredible range of reactions. It's all so splintered compared to what I came to expect in the theater. I mean, you'll get one guy saying that Bryan Brown is sensational, and we made a mistake by shifting the focus to Brian Dennehy as the homicide cop halfway into the picture. Then the next guy can't stand Bryan Brown and wishes Brian Dennehy had been brought in to 'save the movie' even sooner. It's fun to have all these contradictory impressions circulating around your work.

"I was so ignorant and naive when I made 'Independence Day.' Had never heard of high concept, knew nothing about marketing guys and their problems . . . It's always going to be easier to admire the 'good little film' or the 'promising first feature' than it is to sell it."

Mandel, 39, was born in Oakland and raised in New York, where he now resides with his wife and their 3-year-old daughter. A second child is expected next month. Though reading scripts "every day," he is not inclined to commit to a new project for the time being -- in part because of the baby and in part because he still has a picture waiting for release.

"F/X" was actually his third feature; the second, "Touch and Go," spent a year or so in limbo while the producer, Stephen Friedman, negotiated a switch in distributors from Universal to Tri-Star. Written by Ormsby and Harry Colomby, "Touch and Go" is a starring vehicle for Michael Keaton, who plays a professional hockey star who becomes acquainted with a juvenile delinquent and then romantically entangled with the boy's mother, played by Maria Conchita Alonso of "Moscow on the Hudson." Evidently, Mandel is extremely fond of the picture -- and uncertain of its reception.

"In my opinion it's wonderful, and we ought to break a lot of hearts in a very satisfying way. But there will be this difficult shift of tone and emphasis to navigate with the audience. Michael's performance should be a revelation for his fans, and he worked extremely hard. Got himself into terrific shape to begin with. He can handle himself on the ice. That may be one misconception we've got to overcome right away -- Michael's not playing some travesty of a hockey star; he can really play the game."

Mandel, whose New York theatrical credits included productions of "The Cherry Orchard," David Storey's "Life Class," Jules Feiffer's "Knock Knock" and William Inge's "Come Back, Little Sheba" (Washington theatergoers may recall his staging of "Bartholomew Fair" back in the 1970s), would like to arrange a situation that combined theater direction with movie direction, but at the moment he sees far more potential in the movies.

"I'm afraid the theater may really be dying," he broods. "I know that's the oldest of cliche's -- the 'fabulous invalid' and all that. Broadway's been dying for years and years -- since before we were born and our parents were born. Only this time the prognosis might be terminal.

"The difference is that off-Broadway is dying too, and with it we're losing the range of experimental work that used to revive Broadway along with any semblance of an affordable popular theater. I gather it's tough all over, including Washington, which can take some pride in its theater culture. I don't know . . . but it looks like curtains to me. Most of the playwrights I used to know or work with seem to be writing screenplays out in Los Angeles. And the big show out there is what? 'Legends'! I mean, I love those ladies [Mary Martin and Carol Channing] as much as anybody else, but that's not the kind of show you need to revitalize the theater. I think film has become much more exciting."

Mandel says he is eager to see how the movie public greets "Touch and Go," particularly in the wake of "F/X," which has already altered his career in unforeseen ways. "I'm getting offers to direct action thrillers all the time now," he says. "This would have been unbelievable a year ago, but I was just offered Arnold Schwarzenegger's newest. I had to pass, for a variety of reasons, but it's funny -- and flattering -- to find yourself in that position.

"I haven't ceased regarding myself as an actors' director, or a 'relationships' director. But once you've brought off some thrill-packed spectacle, there's a part of you that wants to do it again for the sake of the audience -- painful as it is during the work process.

"Still, it's not very difficult to keep things in perspective. Most of the scripts I see have trickled down a level after being rejected by the really big names, like Spielberg and Pollack and Lumet. Which is the way it should be, since they've established a track record and given us years of pleasure. Fortunately, they don't like repeating themselves either, so it gives the rest of us a break. I'm not sure anyone could get a foot in the door if the established directors could be talked into making the same film over and over."