The show's characters where based on the actual actors playing them and the series pilot wasn't written until all the actors were cast. The pilot episode was largely improvised with the actors making up most of their own dialogue.
During filming of 'Undeclared,' Seth Rogen approached Judd Apatow with his script for Superbad (2007). They read the script with Rogen reading for Seth, 'Jason Segal' reading for Evan, Martin Starr reading for Fogel, and Kyle Gass and 'David Krumholtz' as the two police officers.
Christina Payano made multiple appearances as a student in the floor before she was introduced as Tina. She is seen in the background many times and she has two speaking parts as a loser girl obsessed with Lloyd. Cast and crew enjoyed her so much they decided to make her into a full-time character.
'Jason Segal' and 'Carla Gallo' play a couple where Segal's character is supposedly eight years older than Gallo's character. In real life, Carla Gallo is five years older than Jason Segal. The actors later reunited as an on-screen-couple briefly in Forgetting Sarah Marshall (2008), and in an Easter Egg on the DVD, the two have banter referring to their characters on 'Undeclared.'