I used to communicate with an author who has since passed away, but was always generous with her critques. She used to ask me, "Do you know how to delete words? See that big rectangle with the word backspace on it? Place your finger on it and press down." She also used to say, "This is a great start, what happens next?" after I sent a full manuscript.
She was the one who taught me to write all the backstory I wanted, but to leave it in the hopper with the rest of the cut bits from the first draft.
If a book isn't starting in the middle of an action, even if it's a character being introspective, it's probably starting at the wrong spot. With few exceptions, most books I've hit publish on begin with the second or third chapter even though they all say they start with chapter one.