Both of the terrors that plague Robbie came from Steven Spielberg's own fears as a child: a fear of clowns and a tree outside his window.
(at around 43 mins) When Steve tells the parapsychologists his family members' ages, he says that Dana, his oldest daughter, is 16 and that his wife, Diane, is 32. Many viewers interpret this to mean that Diane was only 16 when she gave birth to Dana. However, the novelization of the film clarifies that Diane is Steve's second wife and that Dana is actually Diane's stepdaughter. In the movie, when telling the parapsychologists about his family, Steve specifically says, "My daughter, Dana, ..." and not "Our daughter, Dana."
Drew Barrymore was considered for the role of Carol Anne, but producer Steven Spielberg wanted someone "more angelic". Barrymore's audition ultimately landed her the part of Gertie in E.T. the Extra-Terrestrial (1982).
James Karen at the time was also the commercial spokesman for Pathmark supermarkets. He received hate mail from people saying they would never shop there again because of his character's treatment of the Freelings.