The creators have confirmed that the mother of Anna and Elsa and the mother of Rapunzel are sisters, which means that Rapunzel is cousin of Anna and Elsa.
That would explain the cameo that Rapunzel and Eugene do in the film (Just after the song "For the first time in forever"
Elsa was to be the villain of the film, in fact the last minute producers decided that it was not the villain since the song of Let it Go was so strong, positive and of personal growth to be interpreted by a villain.
When her Villana status changed, Elsa became Disney's first princess to not be a teenager.
Hans, Kristoff, Anna and Sven were named in honor of Hans Christian Andersen the writer of the story on which the film is based (The Snow Queen).
The most difficult scene to realize is when Elsa builds her castle during the performance of Let It Go
Anna is the first Disney princess to do a duet with the villain.
Olaf's name is inspired by the phrase "Oh Laugh"
The filmmakers invited a real reindeer to Walt Disney Animation studios to observe their physique and gestures, which they later caricatured to bring Sven to life.
Fans have opined in different forums that Olaf's act of melting away from being with Anna is considered an act of true love so the curse had to be broken at that time.
