When you’re making the most-anticipated movie of the spring, you need courage to wander away from the text of a book as well-loved as “The Hunger Games.”

But as Katniss Everdeen would tell you, guts without brains can cripple you in the arena. Director Gary Ross, who wrote the script with Billy Ray and original novelist Suzanne Collins, stays on target as long as he’s faithful to the source. Every time he diverges from it –every time, as far as I recall – the film stumbles and has to write itself.

Katniss, played with maximum effectiveness by Jennifer Lawrence, is the young archer who represents her impoverished district at the annual games, where 24 “tributes” battle to the last-person-standing finale of the nationally televised event.

Ross takes at least a stab at themes Collins has more time to explore on the page: the way reality shows have become ever more degrading, the willingness of our nation to sacrifice its children, the notion that “branding” matters more than substance in our commercialized world. Ideas get compressed to save time, yet they’re present.

But Ross needlessly sacrifices moments that could’ve made the movie much richer. Case in point: the muttations, hideous creatures released by the Gameskeepers to heighten tension. In the book, they have the features of slain tributes, and we’re disgusted that even the spirits of these teens can be manipulated for grotesque audience enjoyment. In the film, they’re just pit bulls on steroids. Two lines of dialogue and some clever computer imagery could have intensified our horror.

Ross has always specialized in emotionally complex stories where someone gets into a dangerous position in a world alien to him. (He wrote “Big,” “Dave” and “Pleasantville,” also directing the latter.) So the first hour of the film, where Katniss takes leave of the few people who understand her and tries to win over crowds in the creepy capital city, feels right.

She gets her drunken mentor, Haymitch (Woody Harrelson), to take her seriously. She develops an ambiguous relationship with Peeta (Josh Hutcherson), the male tribute from District 12, wanting to lean on him for support but aware that one of them must soon attack the other. (Hutcherson handles his emotional moments well, though I never believed he could toss a 100-pound sack of flour.)

Yet the movie eventually has to march onto the field of battle, and then it loses its way. Evil tributes from other districts seem rude, not malicious or terrifying. Ross cuts action scenes clumsily. He gives time to irrelevant people, such as the head Gameskeeper (Wes Bentley) or nasty president of Panem (Donald Sutherland), to underline the social corruption that makes the games possible.

Where Collins’ book paid careful attention to detail, Ross pays far too little. Characters never become exhausted or desperate or gaunt; they don’t even get chapped lips or broken nails. A spear to the chest doesn’t alter a rosy complexion, and Katniss can awaken after a two-day sleep without being thirsty or stiff.

Shouldn’t we feel at every instant that death hangs over these tributes? Shouldn’t we be aware in every scene of the terrible physical and psychological price they pay to stay alive? Instead of a slog through a manufactured hell, the story becomes a kind of Outward Bound adventure with occasional corpses, gussied up for middle-schoolers.

People who haven’t read books two and three may be bewildered by additions, such as a riot that follows the death of a huggable little girl. (The games have gone on for 74 years, and the government’s unprepared for public rage?)

And those of us who enjoyed Collins’ first book without finding it especially deep – a group that includes me – may wonder why Ross abandoned so many things that made it a satisfying journey.