Interview with one of the developers can be found here:
Here are some highlights:
The other 3 characters will continue what you were doing when you switch from them in a mission. For example, if you are assaulting a building, you can put Michael into an overlook position with a rifle, start sneaking Franklin around the side of the building to ambush from behind, and charge in with Trevor on a vehicle, as you switch back and forth between the characters, they will continue what you had them doing.
This can also be used when your In a car chase, you can switch to other characters to shoot pursuing cars while the A.I. Drives
If you leave a car somewhere it will show up at the impound. You can retrieve it for a fee.
Cars will leave petrol trails if there fool tank is shot. This can be ignited to blow up the car.
Car customization ranges from cosmetic grills, tires, etc, to bulletproof windows, armor plates and bulletproof tires.
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