In the Purple Sage, a Treasure Hunt : SILVER LIGHT <i> by David Thomson (Alfred A. Knopf: $19.95; 328 pp.; 0-394-55622-4) </i>

Do the characters from Westerns have a life off-screen? Not just the easy ones, Wyatt Earp and Bat Masterson, the hanging Judge Roy Bean and Billy the Kid and Pat Garrett--everyone knows they really lived, before myth wrapped round them like kudzu.

But how about “Red River’s” Matthew Garth and “The Searchers’ ” Ethan Edwards; how about Ransom Stoddard and Tom Doniphon and Liberty Valance from “The Man Who Shot Liberty Valance,” or James Averill and Nate Champion from “Heaven’s Gate”? Screenwriters created them; actors inhabited them; directors browbeat or cajoled them into their particular vision. Did they fade away when the projectors stopped?

Not for David Thomson. To him they are as real as John Wayne’s sweat-stained Stetson, and their women are too. So for “Silver Light,” Thomson’s intricate, hypnotic history/romance about the Old West and how we have come to perceive it, he dips into the pool of common movie experience and into his own imagination equally, ladling out characters who are a brash mixture of both.

Thus one of the pillars of his story is Susan Garth, who in the 1950s is 80 years old, a possibly great undiscovered photographer. It is her memories that carry the narrative back to the 1860s. She lives in splendid isolation at Arizona’s Canyon de Chelly, near Monument Valley, the most movie-exploited land in the world. And although no one will be blamed for reading a little of Georgia O’Keeffe into Susan Garth’s character, she is actually the daughter of the movies’ Matthew Garth, forever and indelibly Montgomery Cliff in “Red River,” and he will become a central character on these pages as well.

Then there is Susan Garth’s lifelong friend, Bark Blaylock, wholly created by Thomson. He’s a journalist, writer, young witness to the death of Billy the Kid and part-time technical adviser on Hollywood Westerns, who may have been the son of Wyatt Earp. Or Bat Masterson. Or the cowboy “Curly Bill” Brocius. All of them real enough Westerners.

Nora Stoddard, the provocative, prescient young Santa Fe museum curator who discovers Susan Garth, seems to be at the book’s center. Much of the present-day story is concerned with the attraction that springs up between the well-born Nora and James Averill II, the 50-year-old, casually philandering Eastern philanthropist who heads the Averill Foundation, to which Nora applies for backing of a show of Susan Garth’s work.

(And if the first James Averill was played by Kris Kristofferson in “Heaven’s Gate,” Nora carries an equally heavy movie heritage. Her father was the late Sen. Ransom Stoddard, a.k.a. James Stewart, who did or did not shoot Liberty Valance in the movie of nearly the same name.)

To Thomson’s wide-ranging, sometimes arcane cast of film characters throw in Charles Ives, on the occasion of the first performance of his “Three Places in New England”; Willa Cather; alto-sax great Wardell Grey and Erich von Stroheim, hard at work on “Greed” in the furnaces of the desert.

This technique, mixing and matching real and fictional characters, has been a staple of E. L. Doctorow’s; Gore Vidal pioneered a similar sort of legerdemain years ago, but Thomson’s movie-critic credentials lead him to draw primarily from films. He played a simpler version of this game once before, with “Suspects.” There he prowled in and out of the lives of more than 80 movie characters, filling in details of their lives off-screen with the fevered imagination of an over-the-moon movie buff.

“Silver Light” is more ambitious than “Suspects,” harder to sustain, with a panoramic narrative that whips us back and forth in time. For the most part it’s done dexterously, although when one of the characters, referring to some mischief Wyatt Earp is up to, says, “I believe he has done for some of those boys. There are bodies in the corral,” there is an almost irresistible impulse to shout, “OK!!” if only to show that we’re paying attention.

The writing, admittedly, strays into the purple and the unintentionally silly at moments: Averill with Nora in bed, “looking down on the closed eyes of Nora Stoddard, and on the hills of her, all blindly witnessing him,” or 15-year-old Sally Averill hearing her first Charlie Parker, live, in 1950: “She cannot stop moving to the music, keep up, keep on, the beat, bop. She notices that her body keeps the bass time . . . while her fingers and her hands are playing out the notes of the saxophones and the vibraphone.” Busy girl.

However this is more than balanced by the book’s Byzantine and eventually seductive design and characters, so deftly and imaginatively interwoven that finally there is nothing left to do but surrender to them.

To the astringent Susan Garth, and Thomson’s wonderful sequence in an abandoned silver mine in which her own image imprints itself on a piece of silver left behind by a miner--and photography as her calling is as deeply imprinted on the young girl.

To a masterly thunderstorm sequence, after which young Bark Blaylock fords the Pecos river on horseback.

And to a picaresque subplot about Jack Chance, a sweet-natured young middleweight managed by Blaylock, who must be goaded to anger to fight, and Bark’s scheme to get him mad by flirting with Chance’s widowed mother. One can put up with a bit of purple, when the rewards are this good.

Sensibly, the novel doesn’t demand that we have the author’s passion for its movies, or even know all of them. (The discovery of an index at the end of the book to every film character was an enormous relief.) Obviously it makes a novelist’s job easier when a character is called Ethan Edwards, and when John Wayne shimmers to mind, doggedly searching through the years for a child carried off by Comanches.

But if “The Searchers” isn’t in our memory banks, “Silver Light” still works. Thomson remains true to Ethan Edwards, or to Wayne’s image of him as stamped by John Ford, who will flicker into the story briefly too, as one of two curmudgeonly directors, “Jack” and “Howard” (Hawks).

Curious, Thomson follows “The Searchers’ ” characters after the film leaves them. What would have happened to Deborah, the young woman Ethan Edwards rescued after more than a decade with the Comanches, half-deranged with not knowing what people she belonged to. The book imagines Deborah’s later child, beautiful, headstrong and not a little spooked by her mother’s history. This is Sharon Pawley, who will become the love of both Bark and Susan’s life, a natural magnet for a movie camera and the star of Bark’s primitive version of “Tess of the D’Urbervilles,” shot in 1915 in Northern California, with none other than Susan Garth behind the camera.

Can’t remember that silent film? Ah, that’s because of the chicanery of Noah Cross, decades before his appearance in “Chinatown,” in the person of John Huston. It’s another of the interlocking bits that makes “Silver Light” the treasure hunt that it is.

The only thing predictable about the book is that it won’t make a great movie. It already has--about a dozen of them.