“World War Z” may be the first film in which the cast exceeds the actual population of the planet. There are huge citywide vistas of rambling crowds. Most of these people are infected with a zombie virus that turns them into rattlesnakes with overbites and clammy hair.

These elements pay off quickly in a fantastic opener set in a traffic jam in downtown Philadelphia. Amid startling car-smashing, Brad Pitt and the crowd sprint through vehicles like frightened deer. Director Marc Forster (“Quantum of Solace”)–who never lets a shot last longer than an eyedrop— makes the chaos seems freshly organic. That the rest of the film never lives up to this intensity is quite a shame. “World War Z” is a film that promises more than it delivers.

In that opener and other early scenes–such as when the genius doctor chosen to solve the virus mystery gets sent to the showers early–“World War Z” teases you with the idea that it could shatter clichés and be something else, if not something more. Each time, it ends up back in the same zombie chase. After a while, you realize it really isn’t going anywhere except the normal places.

What caused this outbreak of zombie movies? I suspect they started during the pandemic fears around the SARS outbreak of the late nineties. Originally in movies like “28 Days Later” this gave the zombie movies a new pandemic twist: people as incubators for disease. Those twists have congregated into a new normal, with a routine of rules all their own.

Having a star of the magnitude of Brad Pitt laconically swing through these motions represents the simultaneous crowning and bottoming of the genre. There’s something brilliantly metaphorical about Pitt strolling with immunity through a charging zombie wolfpack. In a sense this is no different than the rest of the movie, and points out why the best zombie flicks tend to have unfamiliar actors in starring roles – you have to believe deep down that they really have a chance of being eaten. I also wonder, when did horror films stop developing great scores? Marco Beltrani’s effort is unnoticeably pedestrian, and the cues are so conventionally used.

On a fun level, “World War Z” should be next in line for the hang’em high treatment for the movie logic police–those Internet comedians who go through movies like “Prometheus” shot for shot and debunk the choices made. For instance, if entire cities were taken over by zombies who couldn’t survive fire, why don’t the survivors out on just set fires? Is there a zombie firefighter brigade we’re unaware of?