The song that Jack Black sings at the party was improvised. Jared Hess admitted he didn't have time to come up with one, Jack quickly replied "Leave it to me, I already got one."
During filming Jack Black recorded a series of "confessionals" which were uploaded onto the Internet as video podcasts available from iTunes. They became one of the most popular podcasts in the world.
The film was partly inspired by Fray Tormenta ("Friar Storm"), aka Rev. Sergio Gutiérrez Benítez--a Mexican priest who became a wrestler to raise money for his parish.
While filming a wrestling scene, Jack Black jumped from the ring and struck his head on a chair. This caused a black eye and a gash above his right eye which needed stitches.
During some of the wrestling scenes they used blow-up dolls to replace some of the audience. Supposedly, they cost much less than extras and they looked real.