Factual error: When Janet Leigh is shown lying dead on the floor of the shower, there is a close-up of her open eye. The pupil is contracted to a pinpoint (obviously due to the bright lighting) where it should have been dilated. After the film was released, Hitchcock heard from several ophthalmologists who pointed this out and suggested he use belladonna eye drops in the eyes of "dead" people in future films, as the chemical prevents the pupils from contracting. (00:48:30)
Factual error: The car she trades her car for in at the dealership is a used 1957 car, but when Norman is driving it into the swamp you can see on the license plate that the expiration year is 1956. It couldn't have a tag that expired before its model year. (00:55:25)
Answer: Norman and his mother lived together in the house on the hill above the motel. Norman's mother was such a demanding, controlling, overbearing woman that Norman was eventually driven to kill her. The enourmous guilt of this action, combined with the strain he was already under from her treatment of him, caused him to develop a sort of modified split personality: in addition to the existing Norman, he constantly heard his mother's voice in his mind and even had conversations with it. As time passed, the "Mother" voice in his brain began to have some occasional control over his body. Thus, sometimes Norman was in control, sometimes his mother was in control, and sometimes they could "speak" back and forth within his mind. Norman checked Marion into the hotel, but the sexual attraction caused by her presence made his disapproving Mother personality manifest and kill Marion. Having dealt with the situation, Mother retreated and Norman cleaned up, not remembering. At the end of the film, Mother blames Norman for the crimes because she is once again controlling his life and not wanting to take the blame herself. At the same time, this represents Mother forcing Norman down to some corner of his consciousness or even destroying it altogether, meaning that it is unlikely that Norman will ever manifest control again. This is why we hear Mother's voiceover and not Norman's at the end.
Phoenix