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.
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.
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.
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.