I've heard a few editors say they can tell from the first paragraph if they want to contract the book. Yoowza! Where's my file? Let me sharpen my opening hook?
Sometimes you get a visual of how you want a book to start. I did with my first published book, Storm's Interlude. I've gotten the rights back to it, revamped it, and altered the storyline a little. The book will be included in a romantic bundle from various romance authors, coming out in February. But I did not change the opening paragraph.
Someone swaggered out of the moonlit night toward Rachel Dennison. Exhausted from a long day of driving, she braked and blinked. Either she was hallucinating or her sugar levels had plummeted. Maybe that accounted for the male mirage, albeit a very magnificent male mirage, trekking toward her. She peered once more into the hot July night at the image illuminated by her headlights and the full moon. Sure enough, there he was, cresting the hill on foot—a naked man wearing nothing but a tan cowboy hat, a pair of boots and a go-to-hell sneer.
Other times you put yourself into the character's head at the moment, feel her or his emotion and the opening line just smacks you. In fact, you make it the complete paragraph.
Her new neighbor was a man-whore.
The worst times are when you feel you have to give a bit of background to set the scene or describe a character. I have those times more than the ones above that I seem to luck into. This means writing and rewriting. Pouring through my thesauruses and synonym finder books, searching for the right word and, often it's the simplest word.
They don't come easily. Or quickly. I often spend days on opening hooks to hone them to my satisfaction. Even then, the editor might change it. But it has to be good enough to grab her attention and make her want to read on. Or her pinky finger will edge toward that delete key.