MOVIES ARE fragments, glimpses, incomplete looks into imagined lives. Movie characters have nonexistent pasts and hazy futures; what we see, as Jimmy Stewart once drawled to Peter Bogdanovich, are "little . . . little, tiny pieces of time." But what about those pasts and futures, what happens before the house lights go down and after they've gone up? Did Judy Rogers and Jim Stark live happily ever after, after Rebel Without A Cause? Where did fate take Caspar Gutman and Joel Cairo in their search for The Maltese Falcon? What kind of parents raised Taxi Driver's Travis Bickle? Is there some kind of neo-Platonic shadow world out there where these people who we assume are imaginary live out complex, troubling interconnected lives, only a portion of which winds up on the screen? Wouldn't it be a kick to fill in the blanks, to really know what we only suspect?

David Thomson certainly thought so. A film critic and journalist best known for his eclectic, provocative Biographical Dictionary of Film, he has in Suspects, his first novel, put together what seems like a simple biographical dictionary of fictional film folk. Eighty-five individuals are included, characters appearing in 56 movies, almost all of them of the disturbing film noir variety like Chinatown, Double Indemnity, Sunset Boulevard and Strangers On A Train. But these bios are not straight and narrow affairs; they are the creation of a mysterious, secretive narrator, a hidden (until the very end) presence with a dark story to tell, a story whose fearful pattern only gradually becomes clear.

Writing in spare, almost existential prose, Thomson is so much in tune with these old classics that he does a dead-on job of imagining, almost re-inventing his characters' lives, intricately cross-pollinating them with each other and, in Ragtime fashion, with real world types as well. It is eminently fitting, for instance, that White Heat's Ma Jarrett once worked for Buffalo Bill, that Laura's effete Waldo Lydecker read aloud to Henry James as a youth, that the Sunset Boulevard mansion where Norma Desmond called for her closeup was bought for her by Chinatown's Noah Cross, and that she had a son by Joe Gillis who turned out to be Julian Kaye of American Gigolo.

Thomson also enjoys occasionally postulating that these characters ended their lives in different movies from the ones in which they began, that the Amy Jolly Marlene Dietrich played in Morocco is the same woman as the Tanya she portrayed in Touch of Evil, or, even more complexly, that the Kitty Collins Ava Gardner created in The Killers aged into the Grace Devlin that Burt Lancaster's Lou Guarini lived with in Atlantic City. It's enough to make the head fairly spin.

Sometimes Thomson does tend to get a bit too clever for his own good, for instance having Raymond Chandler in The Long Goodbye run into "a drunk named Firmin" down in Mexico. And his tendency, as a member in good standing of the San Francisco media elite, to sprinkle the book with the names of fellow luminaries like Diane Johnson, Tom Luddy and Herb Caen (spelled Kane, natch) wears a bit. Since so much of Suspect's considerable enjoyability stems from one's familiarity with the movies these characters come from, from our ability to see these people in our mind's eye as we have seen them on the screen, it follows as well that the more of these films the reader has experienced, the easier it will be to get the book's sometimes obscure references and the more he or she will enjoy what Thomson does with the protagonists.

It is undeniable that Thomson has pulled off an engaging tour de force in Suspects, that he truly believes, as his narrator says near the close, that "the screen is like a map for our dreams on which we may always travel, without ticket, tiredness or pain. It is our greatest frontier, like a magic mirror." Yet it is equally true that one puts his book down a trifle disappointed. It's not that Thomson hasn't accomplished what he set out to, that his puzzle isn't elegant enough, but rather that it reminds us that what makes literary and cinematic fiction memorable is what happens in that tiny piece of time that's placed in front of us. Suspects, set almost entirely on either side of that golden moment, is finally too peripheral and uninvolved to matter as much as we'd like it to.