Just watched the first act this time around. As a short film it is possibly the most succinct and terrifying 25 minutes in American horror cinema. (So let’s just pretend the other 70 minutes don’t exist!)
I am aware of Fred Walton’s short film The Sitter which is how he landed this feature, and is structurally nearly identical to this opening. And it’s pretty good (and available on YouTube), but so much of the effectiveness of When a Stranger Calls…