The critic Morris Dickstein once said that Jack Kerouac’s “On the Road” is more important as a myth or a cultural marker than as a novel. Its story of exhilaration and adventure resonates with young readers in the abstract, but few will deny its stretches of tedium, or fail to note the misogyny and narcissism expressed by the misbehaving male characters. Nevertheless, when “On the Road” appeared, in 1957, it unexpectedly helped lay the literary bedrock for an emergent youth culture that was consumed with the lure of authenticity and an aversion to social norms. Because this attitude was in due course thoroughly assimilated by the mainstream, the novel has secured a place in the canon as a profound, if somewhat clunky, articulation of the postwar American experience—“the ragged and ecstatic joy of pure being,” as Kerouac’s alter ego, Sal Paradise, puts it.

Walter Salles’s film adaptation of “On the Road,” which arrives in theatres on December 21st, struggles to do justice to both the myth and the novel. Having directed three prior road movies (including “The Motorcycle Diaries”), Salles is no stranger to the genre’s conventions of open space and intermittent lassitude, but his treatment of the book borders on precious. I found Garrett Hedlund’s teen-idol depiction of Dean Moriarty particularly unsatisfying. A portrait of Kerouac’s close friend Neal Cassady, Moriarty is supposed to be irrepressibly talkative and dynamic, a model of hyperactivity. Aside from occasional eruptions of energy, Garrett’s Moriarty has a cooler disposition, and is at times almost laconic. “On the Road” is Kerouac’s long meditation on his friendship with Cassady; if the book is more myth than novel, Cassady is both muse and demigod. Hedlund’s performance neuters the book’s animating Mephistophelian spirit.

Cassady was a complicated soul whose creative energies found release through an immoderate enthusiasm for sex, automobiles, and drugs. His enduring aesthetic legacy was an incorrigibly hedonistic life that his friend Kerouac and sometime lover Allen Ginsberg transmuted into art. To them, Cassady was a revelation, the consummate hipster-savant. In his poem “Howl,” Ginsberg rhapsodized Cassady “whoring through Colorado in myriad stolen night-cars, N.C., secret hero of these poems, cocksman and Adonis of Denver—joy to the memory of his innumerable lays of girls in empty lots & diner backyards, moviehouses’ rickety rows, on mountaintops in caves or with gaunt waitresses in familiar roadside lonely petticoat upliftings & especially secret gas-station solipsisms of johns, & hometown alleys too.” Appearing two years later in “On the Road,” he was celebrated by Kerouac as having “the tremendous energy of a new kind of American saint,” and then, toward the end of the book, diminished (with emphasis) as “the HOLY GOOF,” a wanderer incapable of fulfilling his obligations as a father and husband.

Years later, Cassady had a brief second act in American literature, depicted as the helmsman of Ken Kesey’s epic 1964 cross-country journey in Tom Wolfe’s “The Electric Kool-Aid Acid Test.” He served as a kind of totem for Kesey’s Pranksters, who drew inspiration from Kerouac’s novel. Wolfe’s introduction of Cassady is appropriately brisk:

Off to one side is a guy about 40 with a lot of muscles, as you can see because he has no shirt on—just a pair of khakis and some red leather boots on and his hell of a build—and he seems to be in a kinetic trance, flipping a small sledge hammer up in the air over and over, always managing to catch the handle on the way down with his arms and legs kicking the whole time and his shoulders rolling and his head bobbing.

Wolfe’s book, published in 1968, just months after Cassady’s mysterious death in Mexico, at the age of forty-one, certified Kesey’s frolic as an American folk tale. Cassady materializes as a ready-made icon, conveniently tracing the bloodline from Beat to flower generation. Last year, the documentary “Magic Trip,” comprised of footage shot by Kesey and his cohorts during their trek, offered a rare glimpse of Cassady: restless and motormouthed, a man whose life had led him, it seemed, beyond the brink of lucidity.

Cassady’s own attempts at writing were rough, but his effusive prose—which can be found in hundreds of letters and an unfinished manuscript—had an impulsive quality, anticipating the vigorous style of gonzo journalists such as Hunter S. Thompson. The first line of a 1953 letter to Kerouac captures this breathlessness: