Two stars steal 'Show' THE LATE SHOW A film written and directed by Robert Benton. At the Sack Cinema 57. Rated PG. By Bruce McCabe Globe Staff ' ' "The. Late Show" is a quite engaging, quite original trifle, It's conceived as a detective story along the classic lines but is taken over by brilliant acting and brilliant direction and it becomes a very nice love story. First things first. "The Late Show" establishes Lily Tomlin as an actress of the first magnitude. Good as she was in "Nashville," she was only warming up for this role, which is that of a California fruitcake named Margo Sperling. Tomlin invests all her genius in this role, fleshing it out into a study in vulnerability. It's the most fully realized characterization of a woman I've seen in ages. In many films, there comes a moment when you're aware that what you're seeing is going to be good or bad. (The indifferent is harder to discern.) In this film, the moment occurs near the beginning. Ira Wells, an aging private eye cunningly played by Art Carney, is leaving the cemetery where he has just buried a friend, another detective (Howard Duff) who was brutally ' murdered. Wells is approached by Sperling who goes into this lunatic request that he hire himself out to recover her kidnaped cat. Cat? Wells makes a simple motion with his hands which conveys perfectly what he thinks of Sperling and her offer. It's so right that you realize immediately you're going to be in good hands. "The Late Show" is best enjoyed as a story about relationships, not as a detective story. Robert Benton, an intelligent, sensitive scenarist and director, isn't any better than most of us at contriving the kinds of plots that depend on double- and triple-crosses for their implications. What he's good at is the gesture, the gesture that conveys reams of intelligence about his characters andz their character. . It's the way Wells approaches a chair with a missing seat in Sperling's apartment, the way he sits on the edge of the chair and tries to pretend the seat isn't missing. It's the way Sperling describes her crazy boyfriend as a "tuna." You wonder what the hell being a "tuna" is like. It's the way a gangster who's being covered by a man with a gun talks compulsively about his shaky marriage he sounds as if he's composing a Dear Abby letter. It should be pointed out that Benton doesn't exactly neglect the prerequisites of the genre. There is a scene involving a refrigerator I can't be more specific without shuddering that connotes Hitchcock at his most wicked best. And Benton has his Raymond Chandler dialogue down cold. "There are a lot of ways to play this but in the game I play you go by the house percentages," explains Wells. Benton even has the temerity to put in a car-chase, the most hackneyed device in today's crime movies, but he gets, away with it by giving it a little twist. The strength of the film is in the chemistry between Carney and Tomlin, however. Carney's Wells is your favorite uncle, a shy, conservative man with a limp and a hearing-aid and an abiding curiosity as to why young women can't wear dresses once in a while. Tomlin's Sperling is a woman forced by circumstances to be on the fringes of life but who still has her dignity and doesn't want anyone to forget it. She's that rare woman who's had affairs but isn't available. Buoyed by their success as a detective team, Sperling suggests that she and Wells team up on a more-or-less permanent basis. Wells can't handle the suggestion because he's always been a loner. By the time he realizes he might need Sperling more than he thinks he does, Sperling isn't sure she wants him. She doesn't want to be "holding up my side of the conversation and yours as well," she explains. The dynamic is like -that in this film: Perceptions change. And that's the charm of it.