According to the fan sites like The Leaky Cauldron and Mugglenet, she is consulted however, the directors/producers/script writers can choose whether or not to accept her suggestions.
One example: For this movie the script writers had originally written one character completely out of the movie. JK Rowling told them that while it ultimately was there choice to write out this particular character, they would have a difficult time explaining certain events in the final movie.
Since I have not yet seen this movie, I cannot venture a guess as to which one character they might be referring to.
This is known as a 'clue' ...
Mundungus?
Stan What’s-his-name?