Rupert Saint Leger inherits his uncle's estate worth more than one million pounds, on condition that he live for a year in his uncle's castle in the Land of the Blue Mountains on the Dalmatian coast. There Rupert tries to win the trust of the conservative mountaineer population by using his fortune to buy them modern arms (from a South American country that has unexpectedly found itself at peace) for their fight against Turkish invasion (the story was written shortly before the Balkan Wars). One wet night, he is visited in his room in the castle by a pale woman wearing a wet shroud, seeking warmth. He lets her dry herself before his fire, and she flees before morning. She visits several more times, all at night, and they hardly speak, but he falls in love with her, despite thinking she is a vampire. He visits the local church and finds her in a glass-topped stone coffin in the crypt. Despite misgivings he declares his love, be she living or undead, and she arranges the marriage in an Orthodox ceremony conducted by candlelight in the church one night, although he still does not know her name, and she says she must still live alone in the crypt for the present.
Soon afterwards, she is kidnapped by a forward party of Turkish troops, and he learns that she, Teuta, is not undead, but the living daughter of the local Voivode, who is currently returning from a visit to America. She had fallen into a trance, and was declared dead, but then revived, and the local leaders and clergy spread a story of vampirism which was more acceptable, after the (mistaken) news of her death, to the uneducated locals than the truth. Living up to this story, she had spent her days in the coffin in the crypt, but during heavy rain when the crypt flooded, came out seeking warmth in the castle in which she had grown up, and knew all the secret entrances, and hence her meetings with Rupert behind locked gates.
Rupert leads a relief force which kills her kidnappers and rescues her. But news immediately arrives that the Voivode has just returned to the country only to be kidnapped by Turks himself. They race back to the coast, and Rupert unloads an aeroplane with a near-silent engine from the munitions ship which has also just arrived, along with sets of bullet-proof clothes. The kidnapped Voivode is tracked to a nearby castle ruin, and Rupert pilots the plane onto the castle wall as if it were a balloon or dirigible, lowers Teuta by rope to her father. He dons a set of the bullet proof clothes which Teuta and Rupert are also wearing, and Rupert hauls both up to the aircraft which he silently flies off. The castle is then attacked by local troops and the Turks defeated.
Teuta subsequently reveals her marriage to Rupert to her father, who welcomes him into the family, and the country.

Question: Why did Teuta sleep in a coffin?
Answer: To keep up the story for the locals that she was a vampire.
Alternative answer: To make people believe she is a vampire.