This is from the IMDB Trivia page for The Silence of the Lambs:
After working with former FBI agent John Douglas for some time, Scott Glenn thanked him and said how fascinating it was to have been allowed into his world. Douglas laughed and told Glenn that if he really wanted to get into his world, he should listen to an audio tape of serial killers Lawrence Bittaker and Roy Norris torturing, raping, and murdering two teenage girls. Glenn listened to less than one minute of the tape, and has since said that he feels he lost a sense of innocence in doing so, and that he has never been able to forget what he heard.