During a short outdoor shot of the front lawn, you hear the sounds of children playing (one of the children shouts something like "I missed you!"). This exact same sound clip is used repeatedly 18 years later in the final exterior scenes of 1973's "The Exorcist."
This movie and the play on which it was based, are loosely based on the experience of the Hill family in 1952. An article published by Life magazine about the play is the subject of the Supreme Court case Time Inc. v. Hill, in which the family sued the magazine for stating that play depicted what really happened.
The music Gail Kubik composed for the picture was considered too modernistic by Paramount's executives, therefore some of it was rescored by Daniele Amfitheatrof. Two years later, Paramount returned the music rights to Kubik and published his suite from the score titled Scenario for Orchestra.