Double Dare (1976)

Martin Ellis (Alan Dobie) is a blocked screenwriter who invites Helen, an actress (Kika Markham), to a hotel in central London to discuss an idea for a play he is writing with her in mind. As they discuss the play, Martin discovers that a businessman and an escort named Carol are sat at a nearby table and appear to be speaking lines from the as yet unwritten piece. Martin becomes anxious at what will eventually become of the girl, already knowing that the play will not have a happy resolution