The surprising thing isn’t that “On the Road” took so long to be made into a movie. Jack Kerouac’s novel was published in 1957. Walter Salles’s film, starring Sam Riley (as Sal Paradise), Garrett Hedlund (as Dean Moriarty), and Kristen Stewart (as Marylou), opens Friday. What’s surprising, perhaps, is that someone bothered to make a movie of “On the Road” at all. So much of the novel has percolated so thoroughly through the culture. Many people who’ve never read the novel feel as though they have, it’s so familiar. It wouldn’t be a surprise if they felt they’ve watched it, too.

“On the Road” doesn’t have much of a plot. It’s episodic — more a set of circumstances and impressions than any sort of well-wrought narrative. The chief circumstance is restlessness. The title tells you that. The characters drive and hitchhike and take the bus, crisscrossing the United States and down into Mexico.