A ventriloquist is a person who can speak or utter sounds so that they seem to come from somewhere else.
Being a writer requires you to be a ventriloquist, too. Why? How? Because you, the author, must make each character sound like someone you are not.
You may not be a fifteen year old drama queen, but you've got to make you 15 year old teen girl sound real.
What? You're not a lab technician, working on a unique case for the FBI? How do you make your MC real and believable? By understanding ventriloquism and putting it to use as a writer.
Creating characters who are 3 dimensional and original. That's ventriloquism in writing, not speaking.
Do you understand ventriloquism? Can you make a character, talk and sound real, without the reader hearing you, the author speak?