I think I was more irritated by Margot Kidder who often plays ditzy roles or used to. Hussey was never a great actress I don't think and Keir Dullea was as stiff as a board as her boyfriend. The voices on the phone were bit freaky as well as the killers eyes.I thought Olivia Hussey was particularly atrocious, I remember seeing her in Jesus of Nazareth and Romeo and Juliet. This was so long ago though that I don't recall whether she's a lousy actress or whether it was just her role in Black Christmas.
Anyhoo, you never do see what the killer looks like, but his crazed eyes peeking through the door and his demented phone calls gave me the willies.
Speaking of willies I rewatched The Deliberate Stranger (1986) about Ted Bundy the other day, I remember when it first aired on TV as a two-part mini series. Back then splitting a 3-hour movie into two parts made sense, but trying to watch it all in one go is a labor. It's so badly drawn out that about halfway through I fell asleep, I was less than wowed by Mark Harmon. He was considered a sex symbol back then but not my type, maybe if he had been I would have tried to stay awake. Heh.
Mark Harmon looked a little like Bundy and I thought the TV series was okay as was Harmon. I thought the two actors that played the main detectives were miscast. Not sure why that TV series of Harmon's is so popular and has been for many years. Go figure. There are one or two good books about Bundy that are much better than any of the docos or tv and movies about him. One or two of the Bundy docos are worthwhile as well and much better than the Harmon mini series.