When the gates open during "For The First Time in Forever," there is a cameo of Rapunzel and Eugene (Flynn) from Raiponce (2010). Rapunzel has short, brown hair and is wearing a purple and pink dress (her celebration dress at the end of 'Tangled'), and Eugene is wearing a maroon vest and a brownish sash. They are entering the screen from the left.
Idina Menzel auditioned for the part of Rapunzel in Disney's previous fairy tale, Raiponce (2010). Even though Menzel didn't get the part, a Disney casting director recorded her audition, and two years later, it got her the part of Elsa.
The characters of Hans, Kristoff, Anna, and Sven are a reference to Hans Christian Andersen, the author of "Snedronningen" ("The Snow Queen"), which is the inspiration for Frozen. Say the names quickly in sequence to hear the similarity.
In an interview on "Fresh Air with Terry Gross," Josh Gad (Olaf) said that when he took his older daughter (then about four years old) to her first-ever movie in a theater (Monsters University), there was a trailer beforehand for Frozen. The trailer didn't contain any of Olaf's lines or songs but did feature the brief sound of Olaf laughing. Gad said that as soon as his daughter heard that laugh, she said, "That's Dada, more Dada." Gad started crying in the movie theater.
According to Jennifer Lee, Anna is 18 years old in the film, while both Elsa and Kristoff are 21 years old and Hans is 23 years old.