Not really a horror actress, per se, England's Kathleen Byron contributed one of the most disturbing and startling "mad" performances on film in Michael Powell's "Black Narcissus." As Sister Ruth, one of the nuns cloistered in a convent atop an ancient mountain in India, she slowly begins to crack under the "strange" conditions found there. The wind always blows, the air is thin from high altitude, and there are memories everywhere. One of the village agents is David Farrar, a "man's man" who stirs feelings of lust in both the Mother Superior (Deborah Kerr) and Sister Ruth. Only Sister Ruth is unable to resist and leaves the Order. The scene in which she confronts Sister Clodah (Kerr) while smartly dressed in "everyday" clothing (complete with very bright Technicolor make-up, quite startling) is just amazing to behold. I don't want to spoil what happens next, but suffice it to say Sister Ruth is an interesting and tragic character, and Kathleen Byron is simply unforgettable in the part.
Byron went on to a long career in both film and television, appearing in Hammer noir, as well as their Twins of Evil, as Peter Cushing's wife. She also had a nice supporting part as one of the faculty wives in Burn, Witch, Burn. Surprisingly, one of her last portrayals was as "old Mrs. Ryan," the wife of the elderly gent who visits Tom Hanks' grave in Saving Private Ryan. Her most famous part, and the one she's known for, is Sister Ruth.
Incidentally, the film ran on TCM tonight. Highly recommended!