"You're starting the story in the wrong place." It's a fairly common criticism, in writers' groups, in slush piles. We all know to begin in medias res rather than ab ovo, but choosing the proper middle is a tricky thing. Because the middle you begin with, that's the key to what the story is about. So of course the Iliad begins with rage, and Hamlet with a question of identity. When you think about it, Sandman cannot begin with anything other than "Wake up sir, we're here."
But the middle, that place where the story begins, it has to move as well. It needs to reach out and grab the reader. "Who's there?" is the opening line of what is perhaps the greatest work of literature in the English language because the "who" is potentially a ghost. Of a dead - murdered! by his brother! who is now sleeping with his wife! - king. This is exciting, and the reader is in. Otherwise, "who's there?" is the second line of a knock-knock joke.
We learn, when we learn to write, how to do this. Write a fabulous opening line. Put the ghostpigs right at the beginning. If you've written a prologue, delete it before you send your manuscript out.
But sometimes, you know exactly where the middle is. The Devil is waiting, contract in hand, in a woman's office. And still, you write, and rewrite, and throw out, thousands of words, because you do not, as yet, have the the key, even though the door to the story stands in front of you. Because what kind of story is this? The kind where she throws him out because the Devil is Bad? The kind where she doesn't believe in Devils, or devils, and he's cute, so what the heck? Or the kind where she, too, commits the sin of pride, and signs the contract?
It's the last one, by the way. I wrote the opening chapter yesterday.