I thought Catching fire was really true to the book, except they left everything about katniss being pregnant in the arena. One thing I've heard about the film being different to the book is this... *Character death spoiler*
Spoiler:
Instead of finnick being attacked by muts, he apparently has been shot to save time:/
Ugh that's stupid af
Spoiler:
they've got two films to fill how can the need to save time
The first films should be the entire political side of it, then the second film should start with them going to the capital (arena) so it's just action from the get go