Within ten minutes of Katniss Everdeen's silhouette, we were brought back into the world of Panem's District 12, and it became clear we were witnessing something rare: An impressively faithful book adaptation, both in spirit and in dialogue. But as with any book-to-film adaptation, Catching Fire made some changes. So let's look at some of them, shall we?
Gale and Katniss' Relationship
We've discussed the ways in which Catching Fire got the love triangle thing surprisingly right, but it is notable the film made some changes to the Gale/Katniss dynamics we saw in the books. Namely because there's a lot more kissing (by which I mean at least one more, which means 200 percent more). Because whereas in the books Katniss does emotionally choose Gale at one point, she doesn't really tell him that — Katniss didn't ace "feelings" day at District 12 Elementary. So in the books they never actually enter into the assumed boyfriend-girlfriend dynamic that we see a bit of in the film. It does add a nicely subtle extra layer, though, when she re-enters the arena and falls right back into her feelings for Peeta.
The First Act in District 12
Speaking of when we spend all that time with Gale and Katniss, that first act as a whole is much more compressed than it is in the book. When Gale's ripped in the book version it's for being caught with illegal game after hunting; in the film version it's for standing up to a guard in the name of saving someone else. Given that Gale is already sorely lacking on screentime, it makes sense to devote what he does have to more firmly establishing the character as one who makes those more active choices.
Also compressed in act one: There's less time analyzing the chill between Peeta and Katniss, less time spent watching those two make out on the Victory tour — although we do get more of a visual representation of the rebellion in that Victory tour montage, which is valuable.
In the book there's also much more focus on the hype over Peeta and Katniss' engagement, which was pushed back a bit in the films. In the book, for example, the wedding dress Katniss wears during her pre-Games interview was one of many she'd already modeled for all of the Capitol, and was therefore one the audience already had a strong emotional connection to — so when she popped up wearing it for the interview it caused quite the commotion.
We're Missing a Few Characters
Twill and Bonnie, two refugees trying to make their way to District 13, meet Katniss in the woods in the books and provide some much-needed exposition on the growing rebellion (and, well, District 13). This is provided in the film through Katniss's eavesdropping on the Capitol security team on the train.
We're also missing Darius, a Peacekeeper from District 12 who's ousted and turned into an Avox for the Capitol. He's a small role, but one that really hits Haymitch, Peeta, and Katniss hard — though it would have gobbled up more screentime in an already long film.
A Whole Bunch Of President Snow Scenes
As with any series in which the prose is written around the thoughts of a single person, the film has the opportunity to explore some scenes we'd never otherwise have access to — save an instance of a weird crossover moment where Katniss finds some pensieve and explores Snow's mind which, thankfully, is not something anyone has been subjected to outside of fan fiction.
In Catching Fire, these extra scenes largely surround President Snow — and some Plutarch Heavensbee — and their deadly plotting. There are also multiple moments between Snow and his adorable granddaughter — which I suspect were added just as much for certain Act 3 Mockingjay plot twist as it was so Snow (and the audience) would be able to see the reach of Katniss's influence even in the Capitol.
Side note: There's also a change in how we find out (or are hinted at) the blood that takes up regular residence in Snow's mouth — in the books Katniss smells it and it forever taints her view of roses; in the film we see a backwash of blood when Snow is drinking champagne.
As we prefaced before, overall Catching Fire was impressively faithful to the books — even where it deterred, it did so in the name of the series' tone and message. In fact, the thinning of certain storylines and the reorganizing of exposition did something of a giant favor to the series: It allowed this film to be solidly Team Katniss, and for the story to be one in which every minute is affected by the stirrings of a revolution.