Gabriel Garcia Marquez said, "One of the most difficult things is the first paragraph. I have spent many months on a first paragraph and once I get it, the rest just comes out very easily. In the first paragraph you solve most of the problems with your book. The theme is defined, the style, the tone. At least in my case, the first paragraph is a kind of sample of what the rest of the book is going to be."
His mind has to be developing the rest of the story as he labors over the opening paragraph.
I seem to write pages and pages that ultimately need to be hacked away until I find where the story starts. But I definitely need to get the opening "right" before I can go forward. I view this as a handicap, but at least one great author seems to understand.
Marquez also said: "Ultimately literature is nothing but carpentry. Both are very hard work. Writing something is almost as hard as making a table. With both you are working with reality, a material just as hard as wood. Both are full of tricks and techniques. Basically very little magic and a lot of hard work involved."