Over 1,000 people auditioned for the part of Damien. The child actor was four when he auditioned, around six or seven during filming. The Director sat him on his lap and instructed him to go mad. He punched the Director in the nads and cut his own face during the instructed fit (gave himself a nosebleed in the process I'm told).
Also remains one of the biggest casting calls in history.
(I met him a few weeks ago so heard this from his sister, who was also in the film).
I love film triv.