Jeremy Shada was chosen to voice Finn after his older brother Zach, who played the character in the pilot, was unable to reprise his role due to his changing voice.
With Jeremy Shada growing older, the creators decided to age Finn a year older every season so Shada could keep voicing the character.
All of Lady Rainicorn's dialogue is written in English in storyboards of the episodes she is in. Her voice actor Niki Yang translates her lines in Korean during voice sessions.
As a running gag, in every episode (except in Food Chain (2014)), a small snail is in frame, waving at viewers. The snail is usually only on screen for one shot per episode.
Unlike most animated series, most of the cast get to record their lines together in one room.