Which one of these would make you read on - A or B?
She clung to sleep tenaciously, wrapped in beguiling dreams. It was explained to her afterwards that they weren't dreams at all, only reality breaking through the days of confusion as she rose from deep unconsciousness to full awareness, but she found that difficult to accept.
With her sharp little face set in lines of dissatisfaction, the twelve year old girl sat up and searched for her knickers among the forest leaves. It had finally begun to dawn on her that sex with Bobby Franklyn wasn't all it could be.
I'd have said B. We start right in there, it's shocking, dramatic and I don't know where it's going next but I want to find out. A doesn't tell me anything about 'her', it could be about anyone, and be set anywhere.
They're actually both from the same novel, The Dark Room by Minette Walters. A is the opening sentences of Chapter 1. Did she - or her editor - think it was a bit dull, a bit nothing? I may be completely wrong but my guess is that Walters decided she needed a more gripping start and provided a prologue to supply one. Because B is the opening sentences of the Prologue and after the unnamed twelve year old girl discovers a body on the next page, we never hear about her or Bobby again.
Now, I'm not saying that you need a prologue, but think of your opening sentences. Would they grab a reader in the same way B does?