Brittany Snow said that Rebel Wilson improvised most of her lines and would go on 20-minute tangents that would have the whole cast and crew in stitches.
Upon seeing the title, Adam Devine thought that the film was a baseball movie, and thus wasn't prepared to sing a song at his audition. The producers told him to sing any song, and he chose the Gli amici di papà (1987) theme song. They were impressed and he was hired as the role of Bumper.
The story line between Bumper and Fat Amy was not in the script. Adam Devine and Rebel Wilson would improvise during their scenes together, and Devine would often try to kiss her. This led to Devine and Wilson to create a backstory for their two characters and their relationship. The filmmakers thought the hostile, sexual chemistry between the two was funny and kept some of the scenes in the film, though according to the two actors, there was a lot that was cut out, including a reference to a one-night stand.
Anna Kendrick and Brittany Snow were completely naked while filming the shower scene. Kendrick stated that although she was initially nervous, the atmosphere had been hot and humid that she got comfortable with being nude around Snow (jokingly saying she had her body "memorized") to the point she even refused to put her robe back on between takes.