Shooting inside a car is simple but tricky. It often needs a bit of lengthy preparations and patience. There are three simple ways of taking a shot inside a moving car:
1) Place the camera inside the car. You can keep it on the rear seats while your subjects are in the front seats and get a nice ‘over-the-shoulder’ shot. Or you can shoot from the front seat while your subjects are in the rear seat and get a mid-shot. You can try placing the camera inbetween the two front seats.
Another method is to seat beside your subject and take a close-up shot. If two subjects are involved than take their shots separately and ask them to assume that they are talking to the other charecter. You can mix this two shots while editing and create the required sequence.
2) The second trick is to attach the camera to the bonnet and pre-focus it. This is pretty effective when only one charecter is involved. But the major dis-advantage is the immobility of the camera and the occasional jerks.You can try a gyro machine if you are able to sustain that dammage to your pocket. The other simpler and costless trick is to shoot the scene in a smooth highway or somewhere like that. As for two charecters, you can use two cameras.
Now comes the question of attaching your camera to the bonnet. I’ve always done this trick by using assortments of wooden block to get the proper angle and attaching it by sticking plaster!!! And it worked very well.
3) The third and the most skillfull way of taking the shot is to attach the camera to the end of a boom( just a firm metallic rod should do) and taking the shot from another car( if you like adventure, you can try a bike). The greatest advantage of this method is the easy movement of the camera. You can take the camera back & forth by altering the relative speed between the two cars. This provides an easy method of adding a punch of glamour to your shot.
For my choice, I’d advice a combination of the first and the third method.