All of the actors took major pay cuts in order to help save money for the budget, in particular, Seth Rogen and Zac Efron.
The entire cast and crew became very attached to the twin baby girls Elise Vargas and Zoey Vargas, who played Stella. The entire cast and crew went to the set on the babies' last day of filming so they could say goodbye to them.
The film was originally written as Mac and his friends going against the fraternity, but after Seth Rogen's wife read the script, she said that his friends would not care about the fraternity war, and suggested writing Mac's wife into the script instead.
According to the actors and Nicholas Stoller, the twin girls who played Stella (Elise Vargas and Zoey Vargas) were incredibly well behaved, and were always smiling and happy. The only time one of the babies became upset and started to cry was when Seth Rogen and Rose Byrne yelled at each other while filming their fight scene.
The baby turning around to look at Mac (Seth Rogen) and Kelly (Rose Byrne) having sex in the first scene of the film was accidental. The scene was originally supposed to end after Seth Rogen turned her around. However, director Nicholas Stoller noticed that the baby kept looking around to watch Rogen and Rose Byrne, so he instructed one of the camera men to film the baby. According to Stoller, if you listen closely, you can hear Rogen and Byrne laugh after the baby turns around to watch them.