LA Times article

[Marianne]

http://www.calendarlive.com/movies/cl-ca-
horn18jan18,2,2944333.story#time 

..."By not having to do all this exposition, I can concentrate on 
character and the psychology of the characters."

At the same time, Cuarón knew he was not at liberty to reimagine
the 
film from the ground up. While he wasn't exactly constrained to 
paint by the numbers, much of his filmmaking canvas already had been 
sketched in.

Even if he wanted to, for example, Cuarón couldn't hire a new 
composer — he was obligated to use John Williams, who provided
the 
score for the first two "Harry Potter" films. Similarly, Cuarón 
couldn't redesign Hogwarts, overhaul Diagon Alley, or recast any 
lead roles (outside of replacing Richard Harris, who played 
Hogwarts' head of school, Albus Dumbledore, in the first two films 
but died in October 2002). And finally, Columbus would be looking 
over Cuarón's shoulder, as he remained in London as one of the 
film's executive producers.

...

"The whole goal of taking a franchise in a new direction is what 
keeps them alive," Cuarón says. "Jo Rowling said to me, 'Don't be 
literal. Just be faithful to the spirit.' You might have hits and 
misses. But it's always going to be fresh."