Jimmy Stewart was traded for Olivia de Havilland so she could play in a little film called Gone With The Wind

2001 -- A Space Odyssey

Have you ever wondered why they came up with the acronym HAL for the space ship's computer? You'll notice that H comes before I, A comes before B and L comes before M. What does that spell? IBM which is, of course, a huge computer company. We're sure you already knew that. Although Kubrick denied it, the name HAL was rumoured to be a cheap shot at IBM by implying that HALs technology was a step ahead.

Associated Press. Sandy Cohen. Nicholas Cage is racing his silver Mercedes through the narrow streets of London when beer kegs suddenly start dropping from the truck in front of him.

But he can't swerve to avoid the exploding kegs. In fact, he can't control anything in the car at all. It's not a nightmare, it's movie magic: Though Cage sits in the driver's seat, the actual driving is being done by a stuntman in a metal rig atop the car for a chase scene in National Treasure: Book of Secrets.

It's a classic car-chase tool, said producer Jerry Bruckheimer, who has learned a thing or two about the art of the chase producing dozens of action films and crime shows.

"They add excitement and we try to amp it up," he said.

Not that those things come easy. Or cheap.

Car chases take months of planning and can cost millions to shoot. The concept for the chase is usually born early in the story's development, said Bruckheimer, whose credits include Gone in 60 Seconds, Bad Boys and Days of Thunder.

While artists build a computer animation of the sequence to tell the team which cars are crashing when, mechanics work on modifying the vehicles to ready them for repeated rumplings.

"Rather than taking a naked car, a new car, and just smashing into it, it will withstand 10 crashes" once it's been reinforced, said Steve Dent, a stunt coordinator for 25 years who oversaw the chase National Treasure.

Door panels are removed and steel is added to the car's frame. Roll cages are built in. Sliding rigs are attached to the wheels to create a drifting effect, he said, "like it's got skates underneath it," And top-driving rigs are constructed so the actors at the wheel are really just acting and not wielding any control over the car.

"It's much safer for them," Dent said.

The cars featured in the chase aren't the only ones modified. Camera rigs are also built onto a "good, strong, normally a Chevy truck," he said, along with high-speed tracking vehicles and a specially outfitted camera bike driven by a "world champion motor cross guy."

Prepping the vehicles can take months. Shooting can, too, depending on the location. It took nine weeks to film the National Treasure chase scene because the crew only had access to the London streets on week ends.

All told, that chase included seven cameras, 80 stunt people, the destruction of 40 to 50 cars and many hours of editing.

"It's really in the editing room that you truly create the chase sequence," director Jon Turtelaud said. "An editor with fresh eyes...can create the chase scene with a pace and level of intensity that can only be done through editing."

Computer-generated effects play varying roles in chase scenes, Bruckheimer said: "It depends on the director."

Though chases can be almost entirely digital creations, computers are most often used to enhance the details: adding smoke to the tires or to replace a windshield that was removed for filming.

"The crashes are all real," Dent said.