My 2004 novel "Cloud Atlas" opens in 1850, with a notary on an island-hopping voyage from the South Pacific to San Francisco. But that narrative gets interrupted by another story, set in the 1930s, about a young composer who finds a memoir written some decades earlier by the notary; which story in turn is interrupted by another, involving a journalist and a physicist, whose letters recount the 1930s narrative; and so on, for a total of six different time frames. In the novel's second half, the "interrupted" narratives are continued, and the novel ends with the conclusion of the 1850s memoir.

This "there-and-back" structure always struck me as unfilmable, which is why I believed that "Cloud Atlas" would never be made into a movie. I was half right. It has now been adapted for the screen, but as a sort of pointillist mosaic: We stay in each of the six worlds just long enough for the hook to be sunk in, and from then on the film darts from world to world at the speed of a plate-spinner, revisiting each narrative for long enough to propel it forward.

Thinking about how a novel's structure must be made "film-shaped" has led me to these habits of successful adaptations.

First, the bagginess of novels becomes cinematic tautness. A novel can afford to take its time; meandering is a virtue. Dickens, Thackeray and their contemporaries had magazine pages to fill and needed a scale as full-grown as that great 21st-century narrative format, the DVD box set. And who wants to read a novel that lacks fallow areas and downtime—and thought? By contrast, a film costs at least $100,000 per minute, and after 180 minutes, the human eyeball is in danger of melting, so it has to deliver the plot more quickly.

Second, suggestiveness in novels becomes exactitude in film. Too much detail clogs text like cholesterol clogs arteries, and three sentences of description per roof/landscape/face are normally ample. The trick is to "stroke" the reader's imagination into life and get it to do the work for you. In a film, however, detail cannot be suggested: It is either shown or it isn't. Something similar occurs to dialogue. There are no readers to "hear" a particular line in their own way. The take used by the director becomes the one final version.