 In the documentary included on the 25th Anniversary Edition, the  actors reveal that in many shots it was not necessary to "act", as  what was captured on film were genuine reactions. For example, Ellen  Burstyn mentions that her scream and facial reaction after being  slapped by Regan, were due to being pulled too hard by a harness.  Linda Blairs screaming was a true reaction to being bounced around  on her bed. Reverend William OMalley recalls that William Friedkin  slapped him prior to shooting and this caused his hand to tremble  while blessing Father Karras.