Anyhoo, I came across how JK Rowling presents a surprise to the reader when it affects her characters - she does so by dealing with the most immediate element, and in the case of Harry looking into the mirror - his reaction. This keeps the reader slightly distanced, as if pushing them away so that they can't see what Harry sees, making them want to know more:
His panic fading now that there was no sound of Filch and Snape, Harry moved nearer to the mirror, wanting to look at himself but see no reflection again. He stepped in front of it.The lady can write. As for the screaming book, that's another example of sudden surprise, but this time rather than dealing with the character reaction, we have the most pertinent element of the shock, that being the scream that breaks the quiet:
He had to clap his hands to his mouth to stop himself screaming. He whirled around. His heart was pounding far more furiously than when the book had screamed - for he had seen not only himself in the mirror, but a whole crowd of people standing right behind him.
He pulled it out with difficulty, because it was very heavy, and, balancing it on his knee, let it fall open.So, surprise or reaction first... works both ways but it's dependent upon the specifics. There's no point in her writing about Harry's reaction to the screaming book before we've read that it's screaming. Similarly, we lose any suspense and / or terror if we see the people in the mirror and not Harry's reaction.
A piercing, blood-curdling shrief split the silence - the book was screaming! Harry snapped it shut, but the shriek went on and on, one high, unbroken, ear-splitting note. He stumbled backwards and knocked over his lamp, which went out at once.
Oh, and as for the mirror itself:
- but propped against the wall facing him was something that didn't look as if it belonged there, something that looked as if someone had just put it here to keep it out of the way.Functional description linked in with character action (it was facing him - relates back to character position so that it doesn't feel as if we've stopped to describe it).
It was a magnificent mirror, as high as the ceiling, with an ornate gold frame, standing on two clawed feet. There was an inscription carved around the top: Erised stra ehru oyt ube cafru oyt on whosi.