The characters are supposedly stuck in the lodge because they are snowed in. However, the height of the snow outside keeps changing dramatically throughout the film. It is extremely snowy in the beginning when everyone arrives, almost snow-free when Papa Doc runs outside in a futile attempt to save his wife's life and finally, modestly snowy again at the end.
Sorrell Booke, the actor who played the psychiatrist Dr. Henry Beckman in the film, is obviously replaced with a stunt double in the first kill scene in it.