In the short story The Chocolate Box, Mrs Déroulard was the wife of Paul Déroulard. She was a wealthy young woman from Belgium whose wealth was very welcome to the ambitious French politician. Among her assets was a house in Brussels. Mrs Déroulard died some time after the marriage after falling down from the stairs in her Brussels house.
For a time, it was assumed that her death was an accident. However, Madame Déroulard, her mother-in-law, later told Poirot that she had witnessed her son pushing his wife down the stairs. Déroulard's motive for killing his wife is not clearly stated in the story but there is some suggestion that it was because Déroulard had become interested in his wife's cousin Virginie Mesnard.
In the 1993 ITV adaptation of The Chocolate Box, Mrs Déroulard is portrayed by Lucy Cohu. Here she is given the first name Mariane. There is a scene of her having an argument about her husband's political views which creates a conflict between her faith and her loyalty to the church and her loyalty to him. The scene of Déroulard pushing his wife down the stairs is also depicted as a flashback, narrated by Madame Déroulard. Here it is suggested that the motive is the conflict between their views on the church.