The famous line-up scene was originally scripted as serious. The day prior to filming the scene was a long, tense day of filming in a prison. Singer decided to take the cast to lunch the next day, the day of the line-up scene, to lighten the mood. The lunch turned into a "lovefest" according to Bryan Singer, with all the actors becoming giddy. The giddyness carried over into the scene in that they couldn't stop laughing and making jokes. Singer was originally frustrated, but then liked the takes so much and the idea that it shows them bonding in-story, he threw the funniest ones into the film.
Actors were directed to ask Benicio del Toro to repeat himself if his line readings were ever unintelligible. At least two of these ad-libs were included in the final cut.
Hockney: ...What'd he say??
In the scene where Redfoot throws a cigarette at McManus, he was supposed to aim for the chest. Peter Greene accidentally threw it right in Stephen Baldwin's face, and Baldwin ran with it.