Ds (just 7) is reading the book at the moment. He will want to watch the film as soon as he has finished the book. I haven't really been keeping an eye on the story so i don't know what happens in it, have just the odd chapter that I read to him and bits that he has read out loud to me/ told me about.
How scary is it? I can see it's a 12 cert. My ds is quite sensitive about the tv (scared of Dr Who for eg and won't watch it now) but he is much more able to watch things when he has already 'seen' them in his head from reading the book...eg Prisoner of Azkerban was fine (although have just looked and that's a PG...)