In the last analysis "The Fan" is no better than its source, a crudely contrived pulp thriller about a celebrated Broadway star who is menaced by a psychotic admirer. Before reverting to sleazy form, however, it demonstrates occasional signs of stylishness.

The most conspicuous sign is the opening credit sequence, a dazzling new prelude that rivals the most sophisticated graphic designs created by Saul Bass for Alfred Hitchcock or by Maurice Binder for the James Bond series. Presumably a collaborative effort between Edward Bianchi and title designer Dan Perri, the main title for "The Fam" dovetails unusually small letting with ominous close-ups of objects apparently found resting on the desk of the title character, whose fingers are seen tapping out a letter to his unlucky idol.

This perusal alternates mementos of the star -- a framed photo, a stack of Playbill magazines -- with glimpses of potential weapons -- a closed pocketknife, a jar full of sharpened pencils, a cake knife. The music subsides intermittently to allow snatches of voice-over narration -- the typist presumably reading his letter aloud. The prose matches the images and music for sinister suggestiveness: "Unlike the others," the voice confides, "I want nothing from you. The only thing that matters to me is your happiness . . .I despise those desparate, pathetic people who intrude on your privacy. I adore you as no other ever has. Or ever will."

An undeniable tour de force, the credit sequence is brilliantly calculated to make an audience sit up and take notice. Moreover, the premise seems eerily topical. Current events have made it inevitable that audiences will associate the character with the young men now in custody for allegedly shooting John Lennon and President Reagan. Indeed, the coincidence so preoccupied Paramount, the disributor of "The Fan," that the company added an elaborate disclaimer to preview trailers at some point, explaining that the screenplay derived from a 1978 novel.

Off to a sensational audio-visual start, the movie gradually decomposes, leaving you with a product as appealing as a rain-soaked newspaper. Bianchi, making his beature debut after a career in TV commercials, notably on the Dr. Pepper musicales, continues to show a promising pictorial flair. Forexample, there's one spectacular overhead shot of the fan, Breen, played by Michael Biehn, seated alone amid rows of park benches. Their disordered geometry suggests the aftermath of a band concert -- and also the deranged state of Breen's mind.

Initially, Bianchi tries to sustain the motif of the credit sequence by alternating scenes of Breen's obscure, solitary existence with the glamorous, bustling, milieu of the star, Sally Ross, portrayed by Lauren Bacall. An early warning signal goes off in your head when Sally, returning to her apartment after the final performance of her latest Broadway hit ("It's Called Tomorrow"), is shown to be Lonely Too, despondent about a recent marital collapse.

The estranged mate -- a movie producer named Jake Berman -- soon turns up in the amiable person of James Garner, and the movie develops a fatal weakness for irrelevant subplotting. The incisive contrasts between the worlds of the potential assassin and the potential victim accumulate yeards of slack while Sally is glimpsed in perfunctory rehearsal scenes for what appears to be an excruciating new musical ("Never Say Never") and in feeble peace overtures to Jake. Meanwhile, her demented fan has taken umbrage at an imagined slight and has begun warming up for murder by injuring Sally's loyal secretery Belle, played by Maureen Stapleton, and one ofthe dancers from the show.

Even with the menace out in the open, the script continues to lose its concentration. The police enter the scene, but the presiding detective, played by Hector Elizondo, is bizarrely ineffectual.

You lose track of Breen while following Sally on unproductive tangents. His characterization is reduced to evocations of other movies about psychos -- there's an image copied from "M," a rewrite of the "You talkin' to me?" sequence from "Taxi Driver," and finally a homosexual murder out of "Cruising" topped off by pyromania.

Consequently, the final confrontation between star and fan becomes a semi-laughable shambles. Breen is supposed to corner Sally in a darkened theater right after the triumphant opening-night performance of "Mever Say Never." For some reason he takes a seat in the orchestra during the finale, arriving just in time to hear Bacall croak a soul-searching ballad called "Hearts, Not Diamonds." (It could use the services of Andy Williams and ought to earn lyricist Tim Rice some sort of medal for surpassing banality.)

"Hearts, Not Diamonds" leaves such a silly atmosphere in the house that Bacall and Biehn seem to enhance it while chasing and wrestling around the deserted auditorium. When the heroine begins talking at her aspiring assassin like a bad editorial -- "Don't you think the world's had enough of this, you animal?" -- the process of disintegration is more or less complete. What started out as a taut thriller has somehow evolved into unintentional burlesque, with killer and victim functioning as a second-rate comedy team.