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."
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.
According to Jack Black on his video confessionals, his stuntman/body double Tom had never been a stuntman before. The production was having trouble finding a stuntman in Jack Black's body size, because most stuntmen are "not his shape". Tom was "just some dude" they found that matched his size.
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.