SISSY SPACEK slowly shakes her head. Her large pale eyes grow impossibly larger and paler. "The people who knew and loved them, they're so adamant," she says, the Texas in her voice emphasizing every syllable. "When we were shooting in San Francisco, someone up in Idaho, he'd only known Neal for two months, but when he heard what we were doing he drove his cycle straight from Idaho to San Francisco just to see us. He was adamant that we do things right."

Neal Cassady and Jack Kerouac. The yin and yang of the beat movement of the 1950s, shock troops of a cultural revolution that is still going on. Kerouac the novelist, the sensitive, easily bruised author of the enduring "On The Road," the seeker after joyous life. And Cassady, the in-the-flesh embodiment of all Kerouac sought, a magnetic energizer whose every action inspired a rush of written words and who, as the driver of The Bus for Ken Kesey and his acid-soaked Merry Pranksters, ended up mesmerizing yet another generation of outsiders.

"If De Gaulle had met Cassady he would have mentioned him in his memoirs," Kesey, author of "One Flew Over The Cuckoo's Nest," says now. "His life was the core of the beat philosophy. You could be rich or broke, high or coming down, and still find something beautific, a thing of beauty, in the situation you were in. If any one person had a command of the vision of the '50s and '60s, this guy did. When he died, it was like hearing the Grand Canyon had closed up."

Real people, yes, but also icons, ideals, archetypes for generations of enthusiastic rebels straining against what they saw as the commonplace limits of life. So when it became known that Hollywood, the Great Corrupter, was in the process of fictionalizing Keronac's and Cassady's lives for a $3.4 million film called "Heart Beat," due for release by Orion next summer, a storm of controversy and anguish soon developed. Even the fact that the film was based on a memoir written by Carolyn Cassady, Neal's widow and the close friend and sometime lover of Kerouac, did not quiet the stir.

For openers, Ken Kesey and the Merry Pranksters, at work on their own documentary about Cassady, wrote a letter threatening a lawsuit if they were mentioned in "Heart Beat." Then poet Allen Ginsberg refused to allow the film-makers the use of his name or his poetry, forcing them to invent a composite poet named Ira Streiker to take his place

"There's an element here of trying to rip off, to siphon off the glamor of Kerouac's myth and reconstruct it according to the lights of Hollywood," Ginsberg says, adding, "That's some kind of weirdness, some sort of chutzpah."

Fellow poet Lawrence Ferlinghetti expressed this in even stronger terms - "Neal and Jack would probably roll over and spin in their graves if they could see the movie" - and added waspishly that, "what they (the filmmakers) did was buy this very minor literary property because they could pick it up cheap and get some authenticity to their project and then rewrite it."

And even Carolyn Cassady, now unreservedly behind the project, had at one time felt a great many qualms about it and is still conscious that, "People are upset with me, they feel I sold their lives down the river."

Yet this is not the entire story of "Heart Beat." For one thing, the people who are making the film - producers Michael Shamberg and Alan Greisman and writer/director John Byrum - claim long-standing interests in the Kerouac-Cassady nexus and say that their concern about doing the film with "intergrity" is second to none.

"This film is not being made because someone said, 'Make this, you'll make a million dollars,'" says Greisman, with Stamberg adding, "We have nothing to gain by sensationalizing this material. If we were willing to make Neal a dope fiend and her a nymphomaniac, we wouldn't have had the trouble we did getting financing."

In addition, the cast of "Heart Beat" - John Heard as Kerouac, Nick Nolte as Cassady, Ray Sharkey as the composite poet and Sissy Spacek as Carolyn Cassady - provides the potential for noteworthy ensemble acting.

"Heart Beat" is but a single aspect of a growing revival of interest in Kerouac. "On the Road," which has gone through 1,400,000 copies in 23 paperback printings, will be published in a definitive, anotated Viking Critical Library edition next year. This year has already seen the publication of "Jack's Book," an exhaustive and fascinating oral biography by Barry Gifford and Lawrence Lee, as well as the mass-market paperback edition of Carolyn Cassady's memoir.

Still to come are at least two more biographies plus the expanded version of Carolyn's opus plus Ken Kesey's film plus a documentary from the National Film Board of Canada plus a potential film of "On The Road" by Francis Ford Coppola. At this rate, even Kerouac-Cassady wallpaper seems hardly out of the realm of plausibility.

Aside from its function as counternostalgia, as a reflex reaction against the glop of "Happy Days" and "Grease," this revival comes, or so Lawrence Ferlinghetti thinks, as part of a realization that Kerouac's work "is getting to be like the last vision of an old America, an America that's practically lost."

Freeways were barely dreamt of in the years between 1946 and 1950 when Kerouac took the cross-country rambles that became the basis of "On The Road." Though cynicism tells us that the book should be outmoded by now, a sad relic of some long-gone age, its pages still reveal an electric freshness, an excitement and wonder at experiencing life as Kerouac tools off in search of adventure in the company of the magnetic Dean Moriarty, otherwise known as Neal Cassady.

"He was simply a youth tremendously excited with life, and though he was a con man, he was only conning because he wanted so much to live and to get involved with people who would otherwise pay no attention to him," Kerouac wrote in " On The Road."

The two had met through mutual friends while Kerouac was a student at Columbia in the mid-1940s. Allen Ginsberg, who met Cassady at the same time, remembers him, a bit poetically, as "the car-thief 'Adonis of Denver' with his head full of philosophy."

Others remember him as instinct in action, and enormously attractive to both men and women. When Kerouac visited him in Denver in 1947, for instance, he was involved in what Gifford and Lee call "a busy quadrangle" that included his then wife, Luanne; his future wife, Carolyn; and Allen Ginsberg.

This was a pull that a beginning novelist like Kerouac could not resist, and Cassady for his part was drawn to Kerouac's discipline and to his then-tyro literary accomplishments. In 1952, Kerouac went to California to live with Neal and Carolyn, by then his wife and soon to be Kerouac's mistress as well.