The scene in the club before the mud wrestling was all improvised by Jake Gyllenhaal and Ping Wu, including the "FIVE HUNDRED DOLLAR!" line.
Danny Trejo loved playing his role because he doesn't get killed and he didn't have to kill anyone else.
Jake Gyllenhaal later said in interviews that the original screenplay was a dark comedy with very odd undertones, actually quite similar to Donnie Darko (2001) in style. Studio interference turned the final film into a typical teen comedy.
The bubble suit was specifically designed to make Jake Gyllenhaal continually fall over. The tumbles he takes are not scripted.