Tressilian suffers from rheumatism. He is also shortsighted.
Tressilian is described as being "a connoisseur of ladies' dress", and he is in the habit of noting and crticizing the gowns of the ladies as he waits at the table.
While waiting at the table at dinner, Tressilian is annoyed with Walter Champion, the footman, because he hands the vegetables before the gravy, and drops a pear. Tressilian later reprimands Walter, and finds him "nearly, if not quite, impertinent".
Tressilian is in his pantry when he hears the crashing of furniture and a scream from Simeon Lee's room. He runs up the staircase, together with other members of the household, to see what is going on.
After the death of Mr Lee, Tressilian tells Poirot that in the old days, when all the family was at home, they were not a very harmonious family. He also tells Poirot about opening the door to Harry Lee, and then feeling that when he answers the door on subsequent occasions, he is opening the door to Harry again, even though it is really someone else.
Tressilian later tells Poirot that one of the heavy cannon balls that stood on either side of the front door is gone, although he was sure that they were both there that morning.
Tressilian tells Poirot that he is too old for his work, and is always going back to the old days, remembering Alfred, David, and Jennifer as young gentlemen and ladies. He says that ever since Harry came home, things had changed, and seemed unreal.
Poirot later explains that Tressilian had felt confused because he had answered the door to three men who resembled each other closely, and who could pass for each other from a distance.
In the 1995 ITV adaptation of Hercule Poirot's Christmas, Tressilian is portrayed by John Horsley. The portrayal is fairly faithful to the original. Here he also confesses to having an unsettling sense of deja vu when he meets the various visitors to the house, George Lee, Harry Lee and tellingly, Superintendent Sugden.