"Frozen Heart", the film's opener, has a lot of lines that pertain to the rest of the film, particularly the complex nature of Elsa's magic and story's theme of love versus fear. Frozen hearts later become a plot device further on in the movie as well.
Beautiful, powerful, dangerous, cold! Ice has a magic can't be controlled! [...] Strike for love and strike for fear! [...] Watch your step! Let it go! Beware the frozen heart!
When Grand Pabbie heals Anna's head wound he notes that "You are lucky it wasn't her heart. The heart is not so easily changed but the head can be persuaded." Later on in the movie, she does get a similar wound in her "heart" (it's unclear if they mean her physical heart or her soul).
Early on, Anna mentions dreaming about getting kissed by a troll. She later shares a kiss with Kristoff, who was raised with trolls.
When Elsa learns of her parents' deaths, her powers react in the exact same way as they do later on during the climax when Hans (mistakenly) informs her that her sister Anna is dead. Namely, she inadvertently makes frost all over the room and snow that levitates in the air instead of falling.
Both the lyrics and visuals of "For the First Time in Forever" foreshadow a number of subsequent events in the movie:
When Anna is day-dreaming about finding love, she acts out her fantasies before various objects around the castle. The first is a statue head of a nobleman that looks evil and sinister, the next is various paintings with peasant men that look honest and friendly. Anna finds two potential love interests: The first is Hans, a prince, who turns out to be evil. The next is Kristoff, a peasant, who turns out to be honest and good-hearted.
She also mentions "a beautiful stranger, tall and fair." "Fair" obviously means "handsome," but it can also mean "light-haired." Kristoff is taller and has lighter hair than Hans. As does Elsa, who can be considered a "stranger" due to her spending most of her childhood away from Anna, and who is a bit taller than Anna and who she also finds beautiful (though obviously in a very different way than she finds Kristoff beautiful).
Among the prescient lyrics are such lines as "For the first time in forever, there'll be music, there'll be light," (She ends up singing a song while feeling light-hearted due to thinking she's found love) "There'll be magic, there'll be fun" (She ends up having fun with Hans and then later finds out Elsa has literal magic), "I won't be alone" (She meets Hans and gets to spend time with Elsa for the first time in years) and "For the first time in forever, I could be noticed by someone" (Elsa), all of which come to pass not just at the party, but at the end of the film, when the two sisters are reconciled, there is "light" both metaphorically (everyone is happy) and literally (the storm is over), everyone has fun ice skating, and Elsa's magic comes into the open.
Also, the abrupt ending for this "I Want" Song: "Nothing's in my way!" (immediately gets hit and knocked over by Hans' horse)Hans later turns out to be the film's villain, and poses an impediment to her dreams and optimistic, idealistic worldview.
Hans's line "I'd like to formally apologize for hitting the princess of Arendelle with my horse, and...for every moment after." foreshadows that he plans to kill Elsa.
Hans is the thirteenth brother. 13 Is Unlucky, anybody? After saying he has twelve older brothers, he then immediately reveals that he doesn't have a good relationship with them. And later on in the film, he's revealed as a villain.
"Love Is an Open Door" very subtly foreshadows Hans's true nature throughout:
When he says, "I've been searching my whole life to find my own place," he gestures outward to the kingdom, foreshadowing his want to steal the throne. While saying this, he isn't looking at Anna, but the kingdom itself.
"I see your face," Anna's counterpoint to Hans's "But with you, I've found my place," hints she only sees the handsome outlook Hans shows to the outside, and not his true sinister self.
He doesn't match Anna's movements in a few scenes, showing that he's struggling to keep up this charade. And indeed, he is always slightly behind Anna in his movements and as he sings, as if trying to keep up his act with her, combined with him almost going off the edge.
When Hans and Anna dance in the 'spotlight', their movements are not actually centered, and when they dance like robots on the clock, Anna can be seen moving slightly faster than Hans.
While balancing on the bridge railing, Hans can be seen trying to catch up with Anna. He does the same when they dance on a waterfall.
Before they gaze up at the stars which leads to the "sandwiches" exchange, Anna climbs onto the roof first, with Hans following behind.
Even the heart-shape they make at the end has Anna making the symbol first with Hans following her motions.
In the line, "You and I were both meant to be", Hans says "You..." while Anna says "...and I", both referring to her. This odd use of pronouns hints that Hans will leave Anna all alone, which he later does when he traps her in a room.
In addition to doors being an Arc Symbol for the state of relationships throughout the film, the phrase "open door" takes on an additional meaning in this song. When Anna sings "love is an open door," it's because she's thinking of love as a close relationship, but Hans is Evil All Along and thinking of opportunity, viewing Anna's feelings as a chance to gain power.
The song itself is sung near the beginning of the movie, rather than near the end like most Disney love songs. "Fixer Upper" is sung near the end of the movie, and it's not until the very end that we learn that the latter was the real love song. Even the simple fact of how this romance and engagement is progressing far too smoothly and too early in the film, something that doesn't tend to happen with the heroines of other Disney movies serves as a subtle tip-off to Genre Savvy viewers.
If you look closely, you can notice Anna's smiles are more genuine compared to Hans', especially during the final close-up of the two before Hans proposes to Anna. This is a clue that his happiness is just a ruse.
As a bit of musical foreshadowing, when they sing the line "Love is an open door" for the second time, Hans comes in on the word "door" a beat after Anna, on a dissonant note. Before Hans' note resolves and is contextualised as a musical "suspension", it sounds glaringly wrong for just a moment.
A visual display of foreshadowing shown during the shot of Anna and Hans' shadows dancing in the lighthouse light on the ship sale: Hans' shadow stays lit the whole time, but Anna's shadow goes completely off-center and almost disappears into the darkness until he pulls her back in. This shows that Anna is an Unwitting Pawn in Hans' plan to usurp the throne from Elsa.
As the snow starts falling, Hans tugs his own coat tighter to him, but makes no move to help Anna feel warmer when she's visibly shivering. It foreshadows his self-serving nature.
Anna and Hans both refer to their sudden romance and engagement as "crazy," Elsa responds to the news of the engagement by saying, "You can't marry a man you just met," and Kristoff is incredulous that Anna "got engaged to someone she just met that day." Sure enough, the engagement doesn't work out. Hans is not what Anna hoped he'd be.
When Hans and Anna start to bond, Hans promises her that he'd never shut her out like Elsa did. After revealing his true colors, Hans decides to leave Anna to die by shutting her in a room.
When the Duke of Weselton admonishes Hans for wasting "trade-able goods" by giving them out to the people during the eternal winter, Hans snaps back at the Duke. But rather than calling the Duke out for his callousness, Hans instead affirms that he is now in charge. This indicates that Hans is not such a Nice Guy underneath it all, and brings to mind his darker agenda.
Elsa sings, "You'll never see me cry!" during "Let It Go". Anna's Heroic Sacrifice at the end finally makes Elsa break down, and she does cry.
If you listen closely, there are wolf howls in the background in the scene where Anna falls off her spooked horse. Presumably these are the same wolves that ambush Anna and Kristoff in the woods that night.
A blink-and-you'll-miss-it example, but on the way to Elsa's palace, Olaf responds to Anna's insistence that there's no reason to fear Hero with Bad Publicity Elsa by gushing about how he thinks she sounds like the nicest, warmest person ever... and inadvertently walks through one of her magic's icicles. "Oh look, I've been impaled." While funny, it does foreshadow Elsa accidentally impaling Anna through the heart with her magic (which nearly kills her), as well as almost (deliberately) impaling the Duke of Weselton's henchmen when they come to kill her. She might genuinely be a nice person, but that doesn't mean she can't also cause harm.
During the song "Fixer Upper", there's this line: "People make bad choices if they're mad or scared or stressed, but throw a little love their way and you'll bring out their best!" While it does refer to Kristoff, it also foreshadows Elsa finally gaining control of her powers after Anna's Act of True Love.
Hans glances upwards before pushing Francis's crossbow up to stop him from shooting Elsa, hinting that he deliberately aimed for the chandelier, hoping it would fall and kill Elsa while making it look like he was trying to save her.
Elsa freezing her shackles until they're brittle enough for her to break with her bare hands foreshadows Hans' frozen sword breaking when it strikes the equally-frozen Anna at the climax of the film. Frozen steel really is that brittle.
When the heroes ride back to Arendelle so that Hans can save Anna, Olaf asks, "Who is this Hans?" If Anna never mentioned Hans in the approximately 18 hours since she met Olaf, maybe she doesn't love Hans as much as she thinks she does...
Olaf tells Anna "Some people are worth melting for" and "Love is putting someone else's needs before yours" while trying to keep her warm. This hints that the "act of True Love" won't actually be a True Love's Kiss after all, but a Heroic Sacrifice. Anna gives up the chance for Kristoff to cure her and sacrifices herself to save Elsa, which ultimately breaks the curse.
The Time Skip since the first film is three years, the same age gap between Elsa and Anna... meaning Anna, who was 18 and is now 21, is now old enough to be crowned as queen.
In the opening flashback, Anna and Elsa's mother sings them the lullaby about Ahtohallan which foreshadows the entire plot.
"Where the northwind meets the sea, there's a mother full of memory." Elsa sees her mother when she reaches Ahtohallan.
Ahtohallan turns out to be frozen - it's a glacier, which is a river of ice. It's located "where the north wind meets the sea" - compare the lyric from "Frozen Heart" about the origins of ice: "Born of cold and winter air and mountain rain combining."
"Dive down deep into her sound/But not too deep or you'll be drowned." Even after finding the truth about her grandfather's actions, Elsa wants to know more and still goes deeper. She follows his image deep down into the pit of Ahtohallan, where she "drowns" by being frozen into ice by the magical glacier.
"Can you face what the River knows?" The "river" referred to later reveals that Anna and Elsa's grandfather built the dam to secretly weaken the Northuldra out of fear of their relationship with magic, and then murdered the defenseless Northuldra leader who tried to reason with him. Needless to say, both Anna and Elsa are horrified by this revelation.
"When all is lost, then all is found." Elsa turns into ice after learning that her grandfather was an Evil Colonialist who built the dam to weaken the Enchanted Forest. Anna learns that too, finds out that Elsa is dead because Olaf dies as well, and nearly despairs. She finds herself all alone, Kristoff having previously taken off without a word, leaving her in the dark as to where he is or why he left. Then, she has to provoke the earth giants to break the dam to right the wrong done by her grandfather, which also means destroying Arendelle. And then, Kristoff returns, Elsa comes back to life, saves Arendelle from the tidal wave, and she and Anna rebuild Olaf. However, the full truth about Runeard was not revealed to Elsa (and thus, Anna) until Elsa went to the deepest part of the chamber and "drowned." Only when all was lost, could the solution properly unfold.
During Anna and Olaf's duet about how certain things will stay the same way, the things they sing about don't stay the same and a few put Olaf in danger. Being together is one of the things they sing about; Olaf dies for a while later on.
Also during "Some Things Never Change"...Anna sings about a stone wall that will never fall, ironically right as a piece of it falls apart while Olaf is on it, and later, the dam falls.
One line that stands out of "Some Things Never Change" is the line "We'll always live in a kingdom of plenty that stands for the good of the many". Considering the state of the country's relative benevolence never came up before, it's a big clue that Arendelle didn't always "stand for the good of the many".
In a Blink-and-You-Miss-It moment during the flashback, Iduna looks uneasy when Agnarr says he never knew who saved his life. It's later revealed that she's the one who did.
"Into the Unknown" has Elsa's magic creating images of the four spirits out of ice particles. But it also has a brief image of Agnarr and Iduna as children, hinting at the later reveal that the fifth spirit sprang from their love.
Also from "Into the Unknown", a line in the second verse goes, "Are you someone out there who's a little bit like me? Who knows deep down I'm not where I'm meant to be?" At the end of the movie, Elsa realizes that she belongs in Northuldra.
During the charades game, Anna comments, "Two sisters, one mind." By the end, Elsa realizes that their sisterly bond created a new bridge to connect the two countries. As she says to Anna, "A bridge has two sides, and Mother had two daughters."
Elsa reaches to part the mist after first taking Anna's hand. When they try and fail to part the mist on the other side, they are no longer holding hands. The only way to stop the mist is by the two sides of the bridge working together.
As shown by their interactions with Bruni and Gale, Elementals can be calmed with the use of other magic. When fighting fruitlessly against the rampaging Nøkk, Elsa creates ice reins and literally tames the wild horse, to the point that they eventually become friends by the end of the film.
Elsa and Anna's game of Enchanted Forest at the beginning of the movie foreshadows the entire plot.
Elsa makes a disgusted face when Anna makes the two figurines kiss, saying that kissing won't save the forest. Not on its own, but the loving marriage between Agnarr and Iduna plays a key role in saving the day, being the reason the sisters are the bridge between magic and humanity and Elsa has powers.
A snowman and a prince are endangered because of a spell. Kristoff gets lost in the woods, and Olaf temporarily dies thanks to Elsa's magic being undone.Kristoff also gets married to Anna, who is crowned queen of Arendelle, making Kristoff her consort.
"They wake the fairy queen who breaks the spell and saves everyone!" The fairy queen figurine also has wings. A sort of fairy queen is key in the climax - Elsa, riding in on the Nokk, uses magic to protect Arendelle from a giant wave. A shot shows her on horseback, with her cape (made from two pieces of ice/fabric) billowing out from her back like wings. She can do this because she was woken when a spell was broken, too, by Anna when she breaks the dam, which not only helps stop the damage her grandfather had done, but breaks the magical barrier separating the two nations and restores Elsa to life.
Anna says, "And then they all get married!" She becomes queen and marries Kristoff, who becomes her consort.
When he and Anna see the dam, Kristoff says it's good that the dam is in good shape because if it broke, it would send a tidal wave so big it would wash away everything on the fjord, including Arendelle. After that, you just know the dam is going down. Anna and Elsa find out that their grandfather was a treacherous, backstabbing SOB who built the dam to weaken the Enchanted Forest. Anna provokes the earth giants to destroy the dam, and Elsa protects Arendelle from the tidal wave with her magic.
When Kristoff and Ryder are talking about speaking for reindeer, they say that it's a matter of feeling the words and letting them come out without overthinking it — which also serves as a concise description of what Kristoff fails to do in every one of his disastrous attempts to propose to Anna. He learns to be more straightforward about his feelings, and when he tells her what his feelings are and what he wants, she joyfully accepts.
When the main group spends time with the Northuldra and with the lost Arendellian guards, all the main characters speak with a different secondary character. Elsa spends her time talking with a Northuldran, Honeymaren, about magic and Anna spends hers talking with an Arendellian, Mattias, about the people back in Arendelle and doing "the next right thing" in difficult circumstances. This foreshadows the ending, where the sisters split up, Elsa staying with the Northuldra (and Honeymaren is even the one who suggests it, too) to explore magic further and Anna returning to Arendelle as a queen working to help the country forward.
In the deepest chamber of Ahtohallan, where Elsa follows the memory of King Runeard, you can see her breath, which has never happened before no matter how cold it was. For someone who has supernatural tolerance of cold, this is an omen of disaster. If that weren't enough, Iduna's voice is heard again, singing "but not too far or you'll be drowned" just as Elsa nearly slips off the precipice accidentally.
of how this romance and engagement is progressing far too smoothly and too early in the film, something that doesn't tend to happen with the heroines of other Disney movies serves as a subtle tip-off to Genre Savvy viewers.