Danny Trejo loved playing his role because he doesn't get killed and he didn't have to kill anyone else.
The scene in the club before the mud wrestling was all improvised by Jake Gyllenhaal and Ping Wu, including the "FIVE HUNDRED DOLLAR!" line.
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 tattoos on Danny Trejo's chest "Debbie" (his wife) and "Danielle" (his daughter) are real, and were crossed over for the movie.