"Bob loosened me up quite a bit; I'm sorry I didn't give in to the impulse to loosen up even more," said Robert Benton, describing his association with Robert Altman, who served as the producer of Benton's "The Late Show," an unheralded, unassuming detective melodrama that has emerged as one of the few appealing movies of the last several months.

"Bob and I had known each other casually," Benton explained during a recent stopover in Washington, "but we were brought together professionally by our agent, Sam Cohn, who runs ICM and happens to be one of the unsung geniuses of the business. Sam had read 'The Late Show' and suggested passing it on to Bob, who wanted to set up productions for his company, Lion's Gate, that would be written and directed by other people.

"I was thrilled, because I admired Bob's work enormously, maybe even indiscriminately. I don't have any critical faculties. I tend to go into a trance when I like something. Now I know Bob himself so well that it's nearly impossible to view his movies objectively. They seem like self-portraits. Take 'The Long Goodbye.' It's not really a detective movie.It's about his own situation, about what can happen when a smart guy tries to buck that Hollywood establishment.

"So I was excited at the prospect of working with Bob, but apprehensive too. 'I'm not sure I can improvise,' I said. He stared at me and said, 'I don't improvise. I just rewrite later than you do."

Born and raised in Waxahachie, Tex., Benton has lived and usually worked in New York City for the last half of his 44 years. Slimly built and neatly gray-bearded, he's a soft-spoken, pleasant, urbane man who makes no apparent effort to impose his personality or opinions. As a director, he must belong to the Quietly Persuasive School.

"The Late Show" is Benton's second feature. The first, "Bad Company," was generally regarded as an interesting failure by critics and largely ignored by the public. Before "The Late Show" Benton enjoyed greater fame as a screenwriter, especially as the cowriter of "Bonnie & Clyde," a collaboration with David Newman, whom he had met and begun working with at Esquire in the late '50s, when Benton was an art director and Newman a staff writer. The partnership, which also produced the screenplays for "What's Up Doc?" and "There Was a Crooked Man . . .," has not been dissolved, although they now work independently as well.

Benton credited Altman with increasing his confidence behind the camera. "Bob said exactly the right things at the right time to reassure or rescue me," Benton recalled. "He never interfered, but he was always available with sound, practical advice when we needed help. Perhaps the best piece of advice he gave me was to think of this as just another movie. If it worked, fine; if not, shrug it off and go on to the next one.

"He's trained himself to respond that way. When he was setting up 'The Late Show' for me and 'Welcome to L.A.' for Alan Rudolph, he was taking a beating on 'Buffalo Bill and the Indians.' Instead of brooding about it, he went ahead with other plans, including arranging '3 Women' for himself. He's determined to make at least one feature a year so that he won't have to shut down Lion's Gate or lay off anyone."

Benton recalled that he began the script without any casting preferences for the role of Ira Welles, the aging detective hero of "The Late Show," but finished wanting Art Carney, who had made "Harry and Tonto." Ira's eccentric client Margo Sperling, played by Lily Tomlin, evidently changed as a result of the casting. "Margo was written as a dizzy kid," Benton said. "I visualizes someone like Candy Clark. The comedy had as much to do with generational as personality conflict. Bob was the one who perceived Lily in the role. It was an inspiration, and the Margo you see on the screen is probably more Lily's creation than mine.

"I wish I'd trusted the characters a little more. The next time around I'll worry less about trying to explain the mystery and simply go with the characters. At the time I felt that plots had been neglected, and I didn't want to make that mistake. Maybe we even thought of it as one of the lessons to be learned from the failure of the 'The Long Goodbye.' When Bob and I first discussed 'The Late Show,' he remarked that he wanted to produce it because if was an opportunity to do 'The Long Goodbye' again without making the same mistakes.

"It was a nice thing to say, and it's also perfectly true that all of us are looking for second chances. I recall someone at a party asking Peter Stone why he was working on some script - I can't remember what - and he snapped, 'Because I'm determined to keep writing "The Secret War of Harry Frigg" until I get it right!'"

Benton plans to direct a murder thriller, "Stab," written with Newman, this summer in New York. Then he'll return to L.A. and Lion's Gate to revive the characters of Ira and Margo, again played by Carney and Tomlin, in a sequel titled "The Late, Late Show." According to Benton, "The story begins soon after they've started a correspondence school for private eyes; they've advertised in places like Popular Mechanics and have about four students signed up."

Benton also plans to direct a love story, from his own script, "about two people who begin as lovers and end as friends." Sooner or later he hopes to explore the subjects of his family, his birthplace and the outlaw mentality in a project tentatively called "The Brothers," which may reunite him with Arthur Penn, who directed "Bonnie & Clyde."

"Arthur and I are fascinated by violence," Benton said, "and there are aspects of my family's history that keep nagging at me. My father's father was a poor Texas farmer who improved his lot around the turn of the century by going into the saloon business. It may tell you a lot about the family to say that he had eight children, all of whom voted for Prohibition.

"Not all those votes were anti-liquor. Two of his sons - my uncles - were primed to go into the bootlegging business. They were voting for their own criminal self-interest. Eventually they died as violently as they lived.Though my father never shared their fates, he idolized his brothers. He was always deeply attrated to their illegality, and there's something about the nature of that attraction that I've got to get at one of these days. I suppose what I'm really saying is that I'm determined to keep writing 'Bad Company' until I get it right!"