A word of advice for aspiring screenwriters: If naysayers declare a book to be “unfilmable,” ignore them.

With the recent “Cloud Atlas,” this week’s “Life of Pi” and “On the Road” opening soon, theaters are filled with adaptations of novels supposedly too monumental, complicated, controversial or dreamlike for the big screen. They join a long line of novel-inspired movies that were impossible to imagine in theaters, until someone put them there.

Some noteworthy adaptations discover daring, innovative ways of tackling difficult material. Skeptics said New Yorker writer Susan Orlean’s meditative work of journalism, “The Orchid Thief,” could never become a film. Then screenwriter Charlie Kaufman turned it into a loopy postmodern story of two screenwriter brothers struggling to turn Orlean’s piece into a slam-bang action movie, and titled it “Adaptation.”

Other film interpretations, like “Moneyball,” jettison a book’s complicated, idiosyncratic parts and shoehorn what remains into a familiar construct.

Some novels are taboo because of worrisome subject matter. Paramount Pictures did not want Alfred Hitchcock to make Robert Bloch’s “Psycho,” based on the ghoulish exploits of Wisconsin serial killer/grave robber Ed Gein. So Hitchcock financed the production himself, retaining all the disturbing elements of violence, sexuality and voyeurism that studio heads opposed. It became one of the most influential films of all time, and Hitchcock’s biggest moneymaker.

Shocking subject matter likewise did not prevent Stanley Kubrick from scoring scandalous hits with “Lolita” and “A Clockwork Orange.”

Under new rules

Movies operate with different ground rules than literature, says Stillwater, Minn., screenwriter Shawn Lawrence Otto. “Most novels move the plot forward, then dive back into character and reflections. In movies it’s always action moving the plot first.”

Readers will grant an author more latitude to create his imaginary world than viewers will give a filmmaker. Otto held that axiom in mind as he wrote the script for “House of Sand and Fog” starring Ben Kingsley and Jennifer Connelly as morally compromised foes whose financial struggle turns tragic.

Otto recast the nihilistic ending of the novel by Andre Dubus III, saying it “wouldn’t have worked in a movie,” and instead offering hope of redemption for Connelly’s character.

“When I see an adaptation that solves problems in the novel, I am inspired and filled with admiration, because I know how hard it is,” Otto says. “They’re telling a parallel story. Dancing around the novel and doing it in a beautiful way. That’s a remarkable thing to see.”

Sometimes even the original author doubts it can be done. Take “Life of Pi,” Yann Martel’s fantastic novel of a shipwrecked Indian boy lost at sea in a lifeboat with a Bengal tiger for company.

“When I was writing the book, cinematic as it was in my mind, I thought this is a completely unfilmable story,” Martel admitted. For a decade, the story stymied such notable filmmakers as Alfonso Cuaron, Jean-Pierre Jeunet and M. Night Shyamalan.

In Ang Lee, whose career spans the rich storytelling of “Brokeback Mountain” and the computer-generated spectacle of “Hulk,” Martel said his novel found a director “who could both understand the story, feel for it and overcome the enormous technical challenges.”

Lee called the project “daunting” and wondered, “How do you make that into a movie?” The visual-effects team used much of the film’s $100 million budget to create a digital-effects tiger of uncanny realism, and the crew built the world’s largest wave pool in Taiwan to stage monumental ocean storms.

Yet the core of the story is Pi’s emotional journey as his faith is tested by hunger and exhaustion and he drifts into dreamlike delusions. “Sometimes it feels like courage, sometimes you see it and despair,” Lee said of the undertaking.

Martel calls Lee “the perfect director. It’s a match made in heaven.”

Capturing the spirit

Less fruitful are movies slavishly faithful to the source’s form but not its spirit, taking as many words as possible from the page and putting them on the screen. That Classics Illustrated style torpedoed the screen version of Ayn Rand’s sociopolitical manifesto “Atlas Shrugged,” making a novel of ideas into a didactic talkathon.

A 1967 version of “Ulysses” tried to preserve the poetry of James Joyce’s language by turning it into an internal monologue of the characters’ thoughts, including the first use of the F-word in a mainstream film. It’s a dated take on a literary masterwork.

Of course, taking big liberties is no guarantee of success. The ambitious “Cloud Atlas” refashioned David Mitchell’s sprawling fantasy, reworking its chronological structure of six storylines spanning five centuries and two planets.

Writer/directors Lana and Andy Wachowski and Tom Tykwer introduced a new narrative that energetically hopped between genres and eras. The film set out to be a monumental mindbender. Critics were mixed, and audiences were thin.

Director Walter Salles traveled a long and winding road to film Jack Kerouac’s definitive Beat generation novel, “On the Road.” The epic road trip’s autobiographical meandering, stream-of-consciousness structure and aimless, drifting, free-loving characters resisted the efforts of such talents as Francis Ford Coppola and Marlon Brando.

Before taking on the project, Salles took five years to retrace Kerouac’s cross-country odyssey with a Super-8 camera in hand, interviewing Beat poets Gary Snyder, Lawrence Ferlinghetti and Michael McClure. Salles calls those patriarchs “the youngest 80-year-olds I have ever met.”

Steeped in the lore of the era, he moved into production confident that he knew and understood his characters. Salles worked again with screenwriter Jose Rivera, an Oscar nominee for their adaptation of “The Motorcycle Diaries.” Rivera rejected a Hollywood three-act structure, preserving the book’s free-flowing, disjointed style.

And when shooting, Salles encouraged his cast (including Garrett Hedlund, Sam Riley, Kristen Stewart, Viggo Mortensen, Amy Adams and Steve Buscemi) to improvise freely. While filming on remote back-road locations little changed from the 1950s, the cast and crew listened to bebop jazz continuously, drinking in its uninhibited energy.

The film honors Kerouac’s restless spirit with an ad-libbing, narrative-free odyssey that never feels lost and never slows down to the posted speed limit. It only took 55 years to film the unfilmable American classic.