The murderer, Mrs. LANCASTER I was wondering if there is any explanation that she appears in so many Christie novels? She was in Sleeping Murder, Pricking of My Thumbs (where she had a prominent role and was not a cameo), and the Pale Horse. Why did Christie incorporate her into so many novels? Is there an explanation? As well, is the short story The Lamp is it the same character? Was it a kind of backstory for this killer? I always found her strange, as she was always sitting down in either a mental institution or a nursing home talking about poisoned milk.