WRITERS of crime fiction are notoriously unlike the characters they create. The scholarly Kenneth Millar who, as Ross Macdonald, writes the Lew Archer mysteries, would himself be unlikely to pursue any pastime more violent than bird watching. Travis Magee's creator, John D. MacDonald, has a degree from Harvard Business and acts it. And Colin Willcox, author of the Lieutenant Hastings series, is just about as mean and menacing as an antique dealer, which is what he used to be.

Yet surely the prize must go to the Detroit writer, Elmore Leonard. The man who has made America's toughest town his special province with some of the toughest crime novels ever committed to paper (among them, Unknown Man No. 89, City Primeval, and his latest, Split Images) is, in person, the proverbial pussycat. Met in his comfortable home in Birmingham, Michigan, he seems boyish--in spite of his graying and thinning hair--and kind of shy. There is no outward suggestion that inside that high-domed head of his percolates a brain that has conceived and birthed some of the most convincing criminal psychopaths around in fiction today. Do the neighbors on that quiet suburban street suspect what goes on in his front study? Probably not.

Even that name of his--Elmore! What kind of a name is that for a tough guy writer? His wife Joan, who sits listening, calls him "El," but he is known to darned near everyone else in Detroit as "Dutch" --after the baseball pitcher. And after all these years he's thinking of signing himself that way on his next book. "But not Elmore 'Dutch' Leonard," he says. "That'd look dumb."

All these years is right. He's been writing novels and getting them published since 1953 and is probably the most successful not-especially-well-known writer in America today. For one thing, he's had two careers in two separate genres. He started by writing westerns: "I was working at an ad agency here in Detroit, and I wanted to write something that wasn't Chevy copy--and I thought 'why not try westerns?' To tell the truth I'd never really been out West then, but I liked western movies, and I had the South in my background--my family's from there, and I was born in New Orleans--and that helped me with a feeling for some of the characters.

"But why westerns especially? Well, at that time, just after the war, it wasn't exactly a field crowded with good writers. But there were a lot of western magazines to sell to then. It seemed like a good place to learn."

And in fact Dutch Leonard learned pretty well. Four of his westerns were made into movies-- The Tall T, 3:10 to Yuma (from a story), Valdez Is Coming, and Hombre (one of Paul Newman's most successful movies ever). But about the time that Hombre was published in 1961, the bottom fell out of the western novel market, and it was suddenly time for him to try his hand at something else. He did, with results that not even he could have foreseen.

Leonard began writing crime novels. They are not exactly mysteries, since one is seldom in the dark as to whodunit, and often the novel is narrated at least in part from the point-of-view of the criminal. But some of his criminals are fundamentally rather moral men-- the businessman in Fifty-Two Pickup, for instance, who takes violent action against blackmailers and gets away with it, and the amiable pair of stickup men men in Swag who do in their partners before they themselves are done in.

In the beginning, his crime novels were not all that much different from his westerns. The Moonshine War, in fact, was kind of a "southern"--sort of a simple shoot-em-up set in the modern South (although peopled with characters of real dimension that all his novels have). But when he started writing specifically about Detroit he achieved a kind of breakthrough in his fiction.

Until he wrote Fifty-Two Pickup, his settings had been purposely rather vague--the generic South or Big City, U.S.A. But this one presented itself to him as a story about his hometown, and that was how he decided to write it. But, as he recalls, "Early in the book I had this simple scene to write--a guy crosses the street at Woodward and Six Mile Road--and I suddenly panic. 'Omigod,' I think, 'I have to describe this and get it right! I never had to before. It was not so much that I was afraid someone would check up on me. It was that I knew I had to make it interesting to people who'd never been here before. Then it occurred to me to tell it from the character's point-of-view, and that's when I began to get my sound."

He has been making Detroit interesting to people ever since. He has begun to win a steady, loyal readership for himself. There is an army of avowed Elmore Leonard fans today--among them, the New York poet Ron Padgett, the old past-master of crime fiction John D. MacDonald, and the pseudonymous Newgate Callendar, who writes The New York Times Book Review's regular crime column. Callendar said of him, "Leonard is primarily an entertainer. But he is one with enormous finesse, and he can write circles around almost anybody active in the crime novel today."

"Yeah," Leonard agrees, "an entertainer is what I am, I suppose. I try to move my story with dialogue. I try to write it so that the reader knows the reactions of the characters without getting the physical reactions. Actually, I try to keep physical description to a minimum. I'm not interested so much in situation as I am in characters. I'm on page 319 of my new book Cat Chaser now--it'll be out in the middle of the year--and I still don't know how it'll end. I've got an idea, but I have to sneak up on it. I very seldom have the conclusion when I write."

Dutch Leonard has sold his share of books to the movies--and then some. He's even done some screenwriting himself (Mr. Majestyk and Joe Kidd were produced from his original screenplays), commuting to L.A. from Detroit for story conferences and rewrites. But movie writing has never satisfied him: "My feeling about Hollywood is that you go out there wanting to do My Darling Clementine--a classic--and it turns out the studio wants you to do something like Gunfight at the OK Corral."

But his feelings about the movies and the novel run deeper than that: "I have to admit that for a long time I used to design my stuff for the movies. But I'd noticed that even though the movies bought my books for what I call my sound, the first thing they tried to do was to get rid of any trace of it. Well, about 1976 or 1977, it occurred to me that I could make a name for myself and not just money. Now, as I see it, there's been a gradual shift in my work, so that there's a little more emphasis on--I don't know how else to put it--the literary side.

"The problem is to get better without imitating the people I enjoy reading. I started out imitating Hemingway, but I integrated him. I enjoy reading Updike, but I can't write like Updike. The stories I write wouldn't allow me to. I want to do my kind of story on my own terms. I've been learning my craft long enough--I've done 21 books. Now I've got to do something with it. If I achieve anything, it won't be out of luck."