Gerald Butler as The Phantom & Emmy Rossum as Christine Daae
It’s a little unfair to discuss this movie against the book as it’s a movie version of the famous musical version of the book. It’s one degree off from anything related to the book, so instead let’s just discuss the movie for what it is, a lovely little mess.
On the whole it’s visually a pretty movie but it made a lot of bad technical and casting choices that make not a good adaptation of the musical adaptation of the book. Let’s just start with the big one, the Phantom. They got very little right about his character. First off the decide to skew younger with casting Gerald Butler and gave him a rock edge. Second the deformity is really minimal. I get that one idea they had is humanize aspect of the musical. Like all the Phantom’s tricks and genius illusions are explainable but in making his deformity which kept him apart and unloved by society no more than a bad sunburn and a slight droop in his eye is childish. Also compared to other singers of the Phantom, Butler doesn’t measure up. So we have a good looking Phantom who can’t sing and the is the major converse of his character.
As far as Emmy Rossum as Christine, I’m not a fan of how she sung the role but as far as looking innocent and doe-eyed she was fine. I do wish they and gotten someone else though but considering the casting on the Phantom especially when Ramin Karimloo (favorite Phantom) was cast in the movie as Christine’s father.
More than the questionable casting this movie despite it’s prettiness make some weird choices. For instance the Marquerade scene is all monochromatic despite the songs saying their are colors everywhere and Christine’s not in costume. Why is her Opera costume Empress Sissi and not like the other costumes prior? I could go on and on and on. Those are little things that take a person out the movie especially when you’re supposed to lots in feels.
The emotions that lost in this movie version are partly being the filmmaker made everything more subtle because it’s a movie. The subtly how the emotions present makes it harder to connect so it really just become boring and with the odd choice and weird casting it become a mess albeit a shallow pretty mess.
as Christine Daae.
and 
I can say I prefer Rossum singing at the lower vocal register which is fine for Esmeralda. I think Rossum could sing to role perfectly well.
, just pick a good actor and imagine them in that movie and they would seem bad. Rossum got some credit back as Fiona in the Showtime show 
which I haven’t seen but I saw her in a clip from it and she was good in it.