According to Piper Laurie, she honestly thought her character in Carrie (1976) was too over the top fanatical to be taken seriously. Director Brian De Palma had to take her to the side and personally tell her it was a horror film and not a “black comedy” as she thought it was. Even so, she would constantly burst out into laughter between takes because not only was her characterization and wardrobe laughable in her eyes, but the dialogue itself was humorous for her. To this day, she still refers to and maintains the movie as a black comedy.
More behind-the-scenes facts at IMDb Trivia: Carrie (1976).