I'm not going to go into all the differences between the novel and the film (there are quite a few), but I have decided to go through some of the larger/more important differences between the two; not all the differences are important per se, but they’re noticeable changes. I’ll put the more important updates near the top of each section.
PERSPECTIVE
The most noticeable difference is the fact that the novel is in the first person and the film is told in the third person. While this isn’t a major difference, it affects how the story is told.
President Coriolanus Snow plays a larger role in the film than in the novel. His speech to the tributes is heard, he is seen conversing with Seneca Crane twice, and is shown in the control room carefully observing Katniss and Peeta's triumphant arrival back in District 12 at the end of the film. In the novel, Snow played a very minimal role until the second installment in the series.
The riot that occurs in District 11 after Rue's death is not mentioned in the novel as Katniss did not know it had occurred. In fact, the rebellion in District 11 isn’t even mentioned until the next novel in the series, Catching Fire. The trigger for the riot is different in the film than it is in the novel; it follows Katniss saluting the camera and the reaction from the people of District 11. In the novel, Katniss only salutes Rue and doesn't address District 11 until they send her a loaf of bread (which doesn't happen in the film).
At the end of the film, Seneca Crane is shown being locked in a bright room with a bowl of nightlock to eat as it was his decision to amend the rules to allow for two victors. In the novel, Seneca and his fate isn’t addressed until Catching Fire where it is implied that he had been hanged.
Aside from the other unnamed tributes from the other districts, Effie was the only other character whose name was not said in the The Hunger Games film, even though she is a constant through the film and her name is mentioned multiple times in the novel.
APPEARANCES The most obvious differences between the film and the novel are the physical differences when it comes to casting. I’m just going to include all of the differences here, even though they’re not all a huge deal.
In the novel, Katniss and Gale are described as having olive skin and black hair. In the film, Katniss and Gale are portrayed by Jennifer Lawrence and Liam Hemsworth, both of whom are fair-skinned and brunette; their eyes are also described as gray in the novel. Also, Katniss is meant to be about 16 years old, and Gale is meant to be 18, though both look older.
The novel says that Katniss is smaller than most of the tributes, but in the film she's taller than most of them, even some of the Careers. In fact, in the novel Clove is said to be bigger than Katniss, but in the film not only is Katniss bigger than her but Clove is in fact smaller than many of the other tributes.
In the novel, Peeta is said to be taller than Katniss, but in the film she is taller. He also has blue eyes in the novel, but they’re brown in the film.
In the novel, Thresh is the biggest, tallest and strongest tribute, but in the film, both Marvel and Cato are taller, the latter bigger and more muscular as well.
In the novel, Prim has blue eyes, but in the film, they are brown-hazelish. Aside from that, Prim’s casting (as well as Rue’s) is the most accurate page-to-screen transition.
In the novel, Haymitch is described as being paunchy with curly dark hair and light grey eyes, but in the film he is slim, with straight blonde hair and light blue eyes.
In the novel, Buttercup is described as being a dirty, yellow color, a color that Prim insisted was an exact match of a buttercup flower, which was how he got his name. In the film, however, he is a black-and-white cat.
DISTRICT 12
Madge Undersee, the mayor's daughter, is not present in the film. Peeta's father is also not in the film; he is mentioned once in regard to Katniss's skill with a bow. As a result of their absence, the concept of class differences within District 12 is absent from the film, as is the Mellark family's interest in the Everdeen family. Without Madge’s presence in the film, the story of how Katniss got her golden Mockingjay pin is changed also.
In the novel, The Hunger Games was mandatory viewing for Panem. In the film, Gale and Katniss have a conversation about what would happen if everyone stopped watching, suggesting that the viewing was not mandatory.
Workers are seen presumably going to the mine on the morning of the reaping. In the novel everyone is required to go to the Reaping, so the workers would not go to the mine at all on Reaping day. Speaking of miners, in the novel there are both male and female miners while in the film, the miners are all male.
In the novel, Haymitch Abernathy falls over the edge of the stage in the reaping ceremony. In the film, he is not shown at the ceremony and is not introduced until the train ride to the Capitol. This isn’t important as much as it is hilarious; it also takes away just how much of a drunk Haymitch has become since winning his own Games.
In the film, Effie states that Katniss is the very first volunteer from District 12, implying that she is the first ever. In the novel, it says that volunteers from 12 are a rarity and there hadn't been one in decades.
In the film Katniss is only permitted 3 minutes to say goodbye to her family, whereas in the novel the tributes are allotted at least an hour to say their farewells.
TRIBUTE TOKEN
As I mentioned above, the mockingjay pin was given to Katniss by Madge in the novel. In the film, Madge doesn't appear at all, and Katniss obtains the pin from Greasy Sae. She later gives the pin to Prim, promising that nothing bad would come to her while she had it with her. Subsequently, Prim gives the pin back to her sister while saying goodbye at the Justice Building stating simply it's "to protect you."
In the novel, each tribute is allowed a tribute token to take with them into the Games. In the film, it is unclear if the concept of a tribute token has been eliminated as Cinna seemingly hides the mockingjay pin within Katniss' jacket and without Madge to explain its origin, it is never specified as a token. Additionally, there is no mention of any other tokens for other tributes, whereas the novel described those of Rue and Glimmer.
THE CAPITOL
This one, for me, is very important and should have been included in the film. In the film, the backstory about the Avoxes is not revealed. However, the red-haired girl from the novel that Katniss and Gale saw one day in the woods asking for their help can be seen in the apartment and is credited as "Avox Girl" in the film. Katniss makes a comment to Gale that the Capitol might "cut out our tongues" if they are caught trying to escape District 12, but in the novel, she isn't aware of this punishment until she actually reaches the Capitol.
In the novel, it takes Katniss several shots to become familiar with the Training Center bow's tighter pull before she puts on an impressive display of shooting by targeting not just the target dummy but also a punching bag and an overhead light. She then looks and finds only a few of the Gamemakers have been watching. In the film, she loses the attention of the Gamemakers when her first shot misses. She collects her wits and uses the second arrow to hit the dummy in the heart, but then realizes no one is watching. Incensed at the Gamemakers for ignoring her, she quickly uses a third arrow to skewer the apple out of the roast pig's mouth. She then places the bow on the stand whereas in the novel she flings the bow and quiver across the floor as she leaves. The Gamemakers' reaction to the trick shot is less extreme than in the novel, which mentions one falling into a punch bowl. This is one difference that I think should have been the same in the novel and the film; it shows that while Katniss is a great archer, even she needs to adjust to a different, heavier bow. It also shows the differences in the districts and their financial statuses. In District 12, the best bow and arrows that most families could afford were made of wood, but in The Capitol they are able to afford bows and arrows made with better, though heavier materials. Any archer who had spent their entire life using wooden bows and arrows to hunt with would need more than one attempt at using a metal, heavier bow and arrow to get used to the feel of it.
In the film Octavia's skin is not green and Venia's hair is not in spikes. This is yet another interesting tidbit that would have been cool to see on screen. The outlandish attitudes and fashion of the people living in The Capitol aren’t focused on too much in the film, so being able to see that class difference – especially in contrast to Katniss’ simplistic look – would have been a good addition.
THE HUNGER GAMES
In the novel, Katniss goes several days without water, actually suffering from dehydration. In the film, although it is difficult to gauge exactly how much time has passed, she finds a source of water relatively quickly, examining moist ground and water-indicating moss. This is an important distinction between the novel and the film. The film goes through the game relatively quickly, but in the novel the game actually lasts for much longer – 18 days.
Peeta's leg is almost completely healed after the medicine is supplied in the film, whereas he remained severely injured in the novel as the medicine did not treat the wound itself. In fact, in the novel, following the Games part of Peeta's left leg was amputated because of his wound from a Muttation, forcing him to walk with the aid of a prosthetic leg for the rest of his life. This was not included in the film.
In the novel, the Muttations were wolf-like creatures that resembled the dead tributes due to genetically engineered DNA. In the film, they were mastiff dog-like creatures with little or no human resemblance. I really didn’t like this change from the novel; I was looking forward to seeing the Muttations from the novel.
In the novel, hovercrafts play a much larger role in society; in terms of the games, though, they picked up the bodies of the fallen tributes. This isn’t something they really show until the second film.
In the novel, when Katniss is looking for Peeta in the stream, she yells out "Peeta!" and Peeta answers "You here to finish me off, sweetheart?", before she finds him wounded. In the film, she just finds him camouflaged when he grabs her boot. This isn’t an important difference, but I think the original reaction from the novel would have been a better choice.
In the novel, Katniss checks Peeta's wound by stripping the lower half of his body to check his thigh. In the film, the wound is visible through a cut in his pants. In retrospect, I can understand why this was changed for the film. Also in the novel, Katniss cleans Peeta by rolling him into the stream and helping him walk downstream and also partially disrobing him, including cleaning his underwear. This part is left out in the film.
In the novel, when Peeta is telling Katniss about his early admiration for her, he also tells her that his father initially wanted to marry her mother but her mother married a different man (Katniss' father). This also explains why Peeta's father promised Katniss that he will take care of Prim when they are in the games while he is giving the cookies to her after the reaping (which is also not shown in the film) — Prim looks a lot like Katniss' mother.
In the film, we don’t get to see a whole lot of Thresh, but in the novel he is a bit more present – not much more, but still a little more. In the novel, his exchange with Katniss is longer, and she specifically tells him that she sang to Rue to ease her passing, and begs him to kill her quickly. This obviously touches Thresh, prompting him to spare her. I wouldn’t have minded seeing more of Thresh in the film.
In the novel, Katniss mentions kissing Peeta many times to manipulate the viewing audience. In the film, other than a quick peck on the cheek from Katniss, we only see them kiss once, and it's implied as being genuine affection rather than an act.
In the novel, Peeta and Katniss are stuck in the cave without food or water for days when rain and thunderstorms prevent them from hunting. As a reward for showing on-screen affection, they receive goat cheese, rolls, fruit, and a tureen of the wild rice and lamb stew Katniss told Caesar Flickerman she liked so much in her interview. The rain stops after Thresh is killed. In the film, there is no rain, no food from Haymitch, and they are able to hunt the day after acquiring the medicine.
In the novel, Cato endures the pain from the Muttations ripping him apart for several hours after being dragged into the mouth of the Cornucopia, and mouths "please" to Katniss, indicating for her to kill him to end his suffering. When she can get a good shot, she shoots him in the head. In the film, Katniss shoots him shortly after he falls to the Muttations to spare him a horrible death. We do hear Cato softly cry "Please" before Katniss shoots him through the heart. This wasn't possible in the novel because he was wearing body armour.
In the film, when it is announced that the rule allowing 2 victors has been revoked, Katniss does not instinctively raise her bow to shoot Peeta before dropping it in shame, like she does in the novel.
In the novel, both Katniss and Peeta are near death from cold and their wounds just after the rule change reverts to a single victor. Peeta, having been bitten in the leg by a Muttation, is in danger of bleeding to death. The idea of double-suicide by nightlock is a desperate attempt to end their suffering either by forcing the Gamemakers' hands or by death itself. In the film, Katniss' suggestion of the double-suicide is more overtly rebellious, although it is clear that she hopes the Gamemakers will prefer two living victors to having none—thereby ending hope in the districts and fomenting civil war. The novel, having the benefit of Katniss' narration, makes it more obvious that she is gambling that the Gamemakers will not allow a double suicide.
In the novel, Katniss and Peeta go so far as to put the berries in their mouths. Claudius tells them to stop and announces them the victors, and Katniss (and presumably Peeta) quickly wipes out her mouth with her shirt before Peeta drags her to the lake to flush out their mouths. In the film, Claudius stops them before they put the berries in their mouths.
AFTER THE GAMES
I mentioned this above, but I’ll mention it again because I feel like this is something rather important that was left out of the films. In the novel, Peeta's leg is amputated and replaced by a prosthetic. In the film, he doesn't lose his leg; he suffers from a bit of a limp for a short while after the Games, but that is the extent of his injuries. In the novel, Katniss becomes deaf in one of her ears after the explosion at the Cornucopia; this injury is fixed after the Games are over. In the film, she simply is shown to have temporary hearing loss for a short time after the explosion, but her hearing returns soon after.
In the film, they never truly address that Katniss and Peeta had to pretend to be in love to gain sponsors. Instead, it was made to appear more like true love, even on Katniss' part. However, whether the film was showing true love or false remains unclear as there are facts to support both arguments.
In the novel, Katniss and Peeta each receive half of the crown. In the film, Katniss and Peeta each get their own crown although Peeta receiving his is not shown. His crown can be seen on a pillow in the background.
The novel makes it more overt that Katniss believes her life to be in danger, even to the extent of expecting "an accident" to occur. This is omitted from the film.
In the novel, Katniss and Peeta get in an argument on the train home about whether Katniss was faking her love for him or not. Peeta is crushed when he realizes Katniss was (mostly) faking for the Games. This argument is not shown in the film though there is a subtle moment of realization by Peeta after she tells him she wants to forget what's happened and he sees how happy she is to see Gale. He joins hands with her and his doing so juxtaposes his doing so on the chariot ride into the Capitol - knowing it's primarily for the audience and not for him.