Probably having fallen in love with music and movies at a young age and then first learning about writing by kind of following the path of writers like Dave Marsh and Lester Bangs and being a rock journalist.
I didn't go to film school so my learning was done out in public and showed up on the screen.
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But the mechanics of learning to 'throw your voice' are pretty simple. Anyone with a tongue an upper palate teeth and a normal speaking voice can learn ventriloquism.