In the Agatha Christie novel "Murder in Mesopotamia", Captain Hastings does not appear. In this adaptation, he appears and is related to William Coleman.
But why would she write threatening letters to herself?
Hercule Poirot: O, I think that would be an assumption too great to make, Hastings, on the grounds of the similarity of the handwritings. But if it were so, it would not be an occurence unheard-of.
Hastings: Pretty ruddy ...
When in the Bagdad hotel Superintendent Maitland and Poirot enter the room with the "dead" body of Joseph Mercado, you can see the actor of Mercado just open his left eye preparing the subsequent close shot of his face.
Thanks to the Archeological Site of Oudhna.