The lead actress carried it.

Considering 80% of the film was her alone in a dark house, this really could have gone south quickly. I've never heard of her before but she did a more than capable job of displaying the right emotions throughout and was downright impressive for an unknown (well unknown to me, anyway). The killer was suitably creepy but without a good lead to terrorize I'm afraid this would have been pretty boring all around. One case where the casting really paid off.