One thing I’ve found that helps sell the composite is to be sure to have subtle (or sometimes not so subtle) motion imparted to the actors in the car. We’ve done that with a couple of grips shaking/jogging the car on the stage while the actors are inside. Another thing is to shoot the background plates before the greenscreen scene. Then, you can have the actors lean slightly one way or the other so their physical action is in sync with the background plate. That way, you avoid seeing a turn in the plate, but no one in the car reacts properly. Also, when compositing, think about moving reflections of branches/leaves/light poles/etc. across the surfaces of the car that may be seen.
John
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