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David gets in/into his car.

- Are they interchangeable and equally natural?

- I'm writing a script, would you choose one of them over the other if the scene was outside the car, and if the scene was inside the car?

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anonymous- Are they interchangeable and equally natural?

Yes, though I prefer 'into'.

anonymous- I'm writing a script, would you choose one of them over the other if the scene was outside the car, and if the scene was inside the car?

What? If the scene was outside the car, there would be nobody getting into the car, would there? If anything, they'd be getting out of the car.

CJ

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anonymous- Are they interchangeable and equally natural?

Pretty much. I feel like "his" moves it toward "into", but I can't say why.

anonymous- I'm writing a script, would you choose one of them over the other if the scene was outside the car, and if the scene was inside the car?

If it's inside, I'm suprised that we are just now learning that it is his car. My choice would be guided strictly by ear, which would depend on many factors like tone, period, situation, and David himself.