A FIRST IMPRESSION of Michael Crichton is bound to be dominated by his height. At 6 feet, 10 inches, Crichton is surely America's tallest feature film director, and his height must confer a certain authority on the set, especially among professions as traditionally undersized as actor and producer. Pictures from the set of his new movie "Coma" includes one irresistible shot of Crichton in conference with producer Martin Ehrlichman, who seems to be standing in a hole.

Crichton imposes control in a casual, leisurely style during conversation, suggesting a pleasant consulatation with a nice, if cagy, young doctor. He was, of course, headed for a medical career before being permanently diverted by the success of his popular fiction, which he began writing in college strictly for money, and by filmmaking, which he began in 1972 with a TV movie, "Pursuit," based on one of his own books, "Binary." He directed his first theatrical feature, "Westworld," a year later.

Despite its success, three years elapsed before Crichton got another movie underway. In the interim he has published three books, a critical appreciation of Jasper Johns and two novels, one of which, the Victorian caper story "The Great Train Robbery," will become his next film.

"I was always interested in movies," Crichton said. "That was the original thing. I imagined being somehow involved in making movies long before I thought of becoming a doctor. Medicine never had the same fascination. When I was an undergraduate at Harvard, anthropolgy interested me more than any other subject. I wasn't emotionally committed to medicine. I entered medical school because I decided that being a doctor would be an interesting job.

"I remember wanting to be a writer when I was a kid, but that got side tracked in college. Although I was doing fast, cheap mystery novels for a paperback publisher, I wasn't thinking of writing as a serious, full-time profession. The greatest experience of my childhood was being taken to my first Hitchcock film, "To Catch a Thief." I realize I was slow to discover this phenomenon. I'd led a rather isolated life as far as movies were concerned. That was 1954, and I was already 12. Still, late or not, it was an eye-opener. I discovered that there was this wonderful, intoxicating world up there. And out there.

"I started asking around and found out that this Alfred Hitchcock, the person responsible for this magic, was someone well-known. As a matter of fact, he did this sort of thing all the time. He even did it for a living. I'm not sure I've ever been the same since. Anyway, "To Catch a Thief" was the key. I doubt if we could presume to reproduce that sort of glamorous stimulation anymore. Where are the personalities to carry it? I don't quite see Robert De Niro supplanting Cary Grant."

Crichton's first exposure to a movie set occurred in 1967. "I was visiting friends in Los Angeles," he said, "and arranged to fulfil my dream of watching an actual Hollywood movie actually being made. Do you remember 'Ice Station Zebra.' That was the one. It was an unforgettable experience. Here was this vast stage at MGM made up to resemble the Arctic or Antarctic. Anyway a huge artificial landscape of ice and snow, all brilliantly illuminated.And nobody was doing anything. There were these hundreds of people, actors and technicians and whatever, all waiting, waiting, waiting. Nothing ever happened while I was there.Maybe nothing ever did happen."

Crichton was still in medical school at Harvard when his science-fiction thriller, "The Andromeda Strain," became a sensational best-seller. It was also the first book he had published under his own name. His mystery-writing pseudonyms were John Lange and Jeffery Hudson. Instead of interning, Crichton accepted a one-year research fellowship at the Salk Institute in La Jolla, Calif. Robert Wise's movie version of "The Andromeda Strain" went into production at Universal Studios while Crichton was still on his fellowship.

"I'd take every opportunity to drive up and watch the shooting," Crichton recalled. "It was a pretty big deal at the time. It was a $6-million movie, which seemed awfully expensive in 1970, when the business was in a slump, and the studio was constructing a lot of elaborate sets. Bob Wise was very considerate and helpful. He let me hang around and soak up what I could. At the same time I was going to parties around town, where I'd invariably meet actors and get acquainted. Not quite consciously I began to file away things I learned at these social occasions, especially the behavior patterns of actors and the kinds of complaints they most often express about directors.

"The thing about directing is that it looked do-able. It wasn't like undertaking brain surgery. You're surrounded by a sophisticated technology, but it doesn't need to be intimidating. There are trained specialists ready to help you, and I think every honest director from Orson Welles down has admitted that you can acquire adequate technical familiarity with the filmmaking process in a few hours.

"What you can't acquire in a few hours or a hundred hours is unerring creative control. To be effective a director must be the person with the most detailed image of the finished film. What happens on any production is that everyone comes in with his own idea. The actors are making their movie, the cameraman is making his movie, the designer is making his movie. You're obliged not to be confused by all this input, not to forget what you had in mind. When you fail to control or resolve all those influences, the picture is going to be screwed up in some way. It will be unsteady. The tone will wobble."

Crichton believes that the study of medicine has been practically useful in his filmmaking career. "Absolutely. For one thing, if you're studying to be a doctor, you're always required to pick up new skills. You learn to manipulate some tool you've never used before or go through some process you've never observed. You get accustomed to concentrating on the way things work and building up a variety of skills. You're geared to assimilate new things.

"At the same time you become accustomed to seeing a lot of people and learning to assess them quickly. After a while it becomes second nature to diagnose people on the street. Every medical student has found himself, for instance, standing in line at the bank and suddenly recommending that the man ahead see his doctor, because you know by his color that something is wrong with his liver.

"It's a more complete, reliable process than intuition. Even if you're not obliged to anticipate illness, the training can still help you anticipate other problems. You sense when trouble is brewing and you acquire a knack for soothing tempers and keeping people happy."

Crichton was born in Chicago but grew up with his brother and two sisters in Roslyn, N.Y., after his father became the executive editor of Advertising Age. Asked if his mother approved of his dropping medicine for the movies, he said, "Of course not. Deep down we all know I'm throwing my life away. But then what's a mother for?"

If all the ingredients jell, Crichton's next production might evolve into a caper melodrama as entertaining as "The Sting." The romantic co-stars are Sean Connery and Lesley-Anne Down, who could give an appreciative director as much to conjure with as Cary Grant and Grace Kelly did in "To Catch a Thief." Crichton's original novel reads like a scenario augmented by period details and digressions, most of which should disappear into "touches" and the settings. At any rate, "The Great Train Robbery" is scheduled to begin shooting in Ireland in April, and the proof of this material should be in the filming rather than the writing.

"I'll have Geoffrey Unsworth and his crew," Crichton said, "so I'll be in good hands. The schedule is very tight, though, for a movie requiring so much period work and physical action. We've got 10 weeks, working 6 days a week, so I'll have to push it. I don't know if the story will have a sense of urgency, but the production sure as hell will.

"The work of directing is a function of time. I had only six weeks on 'West-world' and 12 on 'coma,' and I think there's a marked improvement in my work because of that. I feel more comfortable about what I'm doing, and I'm getting better. Some people complain that 'Coma' is too careful and restrained, but at this stage I can't be certain how justified the criticism is. I was hoping for a gradual acceleration of suspense. After holding back in the first half of the film, we try to break out in the second half. Even the music is concentrated in the second half. Before that there's nothing but source music.

"Maybe it's not a complete breakout, and it could be a feature of the way I am. In one sense I think I'm inclined to be a minimalist. I'd like to achieve effects with the least technique possible. I'm more likely to err on the under-shoot side of things. On the other hand I know I'm attracted to attempting more exciting effects. I suspect that attraction will grow stronger with each new movie."