AS AN INCONSPICUOUS screenwriter in the employ of George Lucas and Steven Spielberg, his celebrated friends and patrons, Lawrence Kasdan has already been associated with back-to-back A blockbusters. He wrote the screenplay for "The Empire Strikes Back," the runaway box-office champion of summer 1980. He also wrote the screenplay for "Raiders of the Lost Ark."

This weekend Kasdan himself emerged from the shadow of "Empire" and "Raiders" to establish a distinctive filmmaking identity as the writer and director of "Body Heat," a remarkably assured first feature. Where did that assurance come from? It's a question that the star of "Body Heat," William Hurt, kept reiterating when he and Kasdan spent a day being interviewed by the movie press in New York prior to the film's national release.

According to Kasdan's modest recollection, it represented the culmination of "15 years of being preoccupied with making movies and going to movies every day before I ever sold my first script," plus whatever pointers could be picked up from random observations on the sets of "Empire" and "Raiders." The hard part was getting the opportunity rather than taking advantage of it.

"I worked out a strategy," recalled Kasdan, a husky, bearded, pensive young man who presents a compact, square-cut physical appearance, short of height but broad of brow, face and torso. "Everybody wanted me to write stuff after 'Empire' and 'Raiders,' but I said I wanted to direct and kept insisting on it. George had said to me, 'The only way you can doSee KASDAN, K12, Col. 1 KASDAN, From K1 it is to force your way into doing it.' After paying your dues and demonstrating that you can write, which is the most direct route to filmmaking, there's comes a point when you're obliged to dig in your heels about one special script. In my case that was 'Body Heat.' I was ready to do it and prepared to hold out indefinitely if that was necessary."

In fact, Kasdan also came highly recommended for the opportunity by Lucas, who enjoys considerable behind-the-scenes influence with the executives of the Ladd Company, who were running production at 20th Century-Fox when "Star Wars" was given the green light in 1976. "Body Heat," an updated murder melodrama in the stylishly tawdry tradition of "Double Indemnity," "Out of the Past," "Criss Cross" and many other savory crime thrillers of the '40s, was acquired by Alan Ladd Jr. and his associates when they were still at Fox. When they left late in 1979 to form an independent production company, the property lingered at Fox for several months. According to Kasdan, he resisted repeated suggestions that it be cast with presumed box-office stars. Eventually, Fox gave up and the Ladd group reacquired the project, allowing Kasdan to realize it as he had envisioned in the first place.

The greatest obstacle to his breakthrough was the inertia generated by any major film production. "The killer thing about movies," Kasdan said, "is that priorities are skewed to dealing with practical problems -- stretching cables, mounting lights, loading and unloading the trucks, getting from one place to another, all of them adding up to an enormous confluence of time and money conspiring to sap your energy and drag you down, until getting anything on film may seem like a heroic accomplishment.

"Did someone say poetry is emotion recreated in tranquility? Well, if so, the scriptwriting stage is analogous. The script is done in contemplative solitude, but the moviemaking process is chaos. It becomes your obligation to impose some kind of effective control on that chaos. As a first-time director, I could have been overwhelmed. It's very easy to become caught up in the physical problems and lose sight of the story you started to film. For instance, it requires a sustained effort to do a scene that takes place on a hot, dry day when your location insists on remaining cold and wet. Once those trucks roll and all that machinery is set up, they take on an importance all their own.

"Bill Hurt was an enormous help to me in those circumstances," Kasdan continued. "He was always determined to focus everyone's concentration on the scene to be played. And in most cases you do that one scene that day only. That's why it's a suicidal process unless you're well-prepared. Although Hollywood scheduling mitigates against rehearsal time, it may be the most valuable hunk of time on a picture. Our two weeks of rehearsal made it clear to me that this script I'd dreamed up alone meant all sorts of things to other people. Nobody saw it the same way. The whole thing had left my head and taken on a separate existence. What Bill or Kath (leading lady Kathleen Turner) saw in the characters was frequently different from what I'd intended or imagined. The rehearsal period has to accommodate all that fresh creative input and still allow you time to resolve the differences that arise.

"The great thing is, fertile ground gets better and better and richer and richer. If you can lay down a strong, detailed texture in a script, people are going to be inspired by it. Not only the actors. The technicians in Hollywood are absolutely incredible. They're used to performing specialized skills with one hand tied behind their backs, but they're proud and imaginative too, and they can attune themselves to whatever line of energy a director generates. If you can rally all that skill and enthusiasm, you've got unlimited resources."

Now 32, Kasdan was raised in West Virginia and Michigan. He attended the University of Michigan, graduating in 1970 with a master's degree in education. He worked as an advertising copywriter in Detroit and Los Angeles while trying to sell his original screenplays. The first to be purchased, "The Bodyguard," an adventure melodrama written in collaboration with a younger brother, was the sixth original he submitted through his agent to the studios. Although it still hasn't been filmed, Kasdan thinks it may be due.

He made his biggest impression on people in Hollywood with "Continental Divide," a romantic comedy in the Tracy-Hepburn tradition purchased by Universal in 1977. I first became aware of Kasdan when Spielberg mentioned the script early the following year. At the time he appeared eager to produce it. In the same conversation he happened to mention that Lucas wanted him to direct some lavish new adventure movie that he'd cooked up. Eventually, Spielberg recommended Kasdan to Lucas as the writer for that project, which evolved into "Raiders," and Kasdan was subsequently hired to complete the "Empire" script, left in a state of disarray by the death of Leigh Brackett, the original screenwriter. "Continental Divide" finally reaches the screen this fall. After four years of debate at Universal about casting and directing prospects, it was filmed by Michael Apted, fresh from "Coal Miner's Daughter," with John Belushi and Blair Brown in roles originally earmarked for Robert De Niro and Jill Clayburgh.

Before beginning his second feature, which he describes only as "a comedy written with a woman writer from television," Kasdan will complete the screenplay for "Revenge of the Jedi," the third movie in the "Star Wars" series, scheduled to begin production this January in London. According to Kasdan, he was writing "Raiders" 20 minutes after leaving his first meeting with Spielberg and Lucas, then hired to work on "Empire" upon delivering the first draft of the "Raiders" script to Lucas in San Rafael, Calif.

"I said, 'Wait a minute,' " Kasdan said. " 'What if you don't like the draft?' George shrugged and said, 'I don't think there's anything to worry about. I get a feeling about people. But okay, I'll read it tonight and call off the deal tomorrow if I have to.' That's the way he operates. It's more intuitive than anyone would imagine."

Kasdan said he entertained no desire to direct one of "Star Wars" sagas but would enjoy taking a crack at a later adventure of Indiana Jones. "My interests are more of this world," Kasdan said. "You can take Indy anywhere, and believe me, before we're done he'll have been everywhere. I think there's already a lot of me in 'Raiders.' I wasn't all that familiar with the serials and the comic books that George and Steve had in mind. What I had in mind was the games my brother and I used to play with friends when we were kids in West Virginia. I worked in a lot of that play. That's what it's about as far as I'm concerned -- boys roughhousing, playing games like king of the mountain.

"I didn't think I'd be doing 'Jedi,' but George has been very good to me, and it's a crucial one. I know George's whole public stance is deflation, so it's normal to hear him quoted as saying that everyone may be terribly disappointed but we're locked into the story as originally conceived and it's too late to do anything else. I think there's a certain segment of the public who could feel disappointed -- people who've become so totally wrapped up in 'Star Wars' that any resolution you dream up is bound to come as a let-down. I think that group might ask, 'Is that all there is?'

"But I don't think there's any cause for normal people to be apprehensive. 'Jedi' is gonna be funny and fast-moving and terrific and emotionally satisfying. I think George is a genius, and this need he's fulfilling, which is really a response to his own needs, is enormously important for children. And adults, too. I don't live and breathe 'Star Wars,' but I think it's plenty neat."