I have a scene in a car. Would it be an internal or external shot? I suspect external because the car is outside.
This is always an open question because it's really very much a production issue. Whether a shot is listed as INT or EXT depends on where the the physical action of the scene takes place.
An interior, say an apartment or a house or a mine -- may take place in a real house or a real apartment or a real mine -- or it may be a set.
An exterior generally presumes that you're actually going to go to some place outside. So "EXT., CAR ON HIGHWAY -- means that you're packing up the crew and the talent and taking them, along with the car, to a highway somewhere.
Now, you may put the camera on the hood of the car, or mount it on the side of the car and shoot in through a window -- but it's all still an Exterior because you're all still outside.
Now, what if they shove the camera from outside the window inside the car, or if you're covering action sometimes from outside -- the car turns sharply and dodges another car, while inside, the driver is fumbling around for something in the glove compartment.
For that matter, what if you're shooting inside a house and the actors walk from inside the house out into the back yard?
Well, then you'd do what I often do in scenes like this, which is to simply write: INT./EXT. THE CAR ON THE HIGHWAY
And leave it to production to figure out just what aspects of the scene are going to shot where, when, and how.
In a larger sense, they may just put the car on a set wrapped around with a green screen and do it all as an effect, but that could be true of almost anything. And at a certain point, it goes beyond what you have to worry about.
If you never go inside the car, just use EXT.
If you never go outside of the car, just use INT.
If, as is most often the case, you tend to move from the inside to the outside of the car over the course of the scene, use, INT./EXT.
NMS