Question: My book is about a Plague Doctor who is in a large city that has been placed under city wide quarantine. He goes around doing what is needed and such, but he never talks. He sighs, grunts, groans, etc. But he never talks. He is not mute, obviously. How can I make it normal that he doesn't ever talk?Answer:
It's not normal, but isn't that the point?
It seems to me that you are deliberately creating a mystery for the reader, which is not a bad thing. Readers may grow curious about this mysterious doctor and why he doesn't speak, and that may keep them reading.
Of course, at some point it would be merciful for you to give some little reveal, some explanation for his choice. Does he not speak the local language? Does he prefer to distance himself from people? Is he suffering from a kind of brain damage that impairs his speech centre (but not his skills as a doctor)? Is it a philosophical choice?
I suspect you would have an easier time writing this story from the point of view of the doctor, so the reader may be privy to his thoughts and feelings, if not his words.
But he would make a fine impact character too. Perhaps you would prefer someone else to be the main character who encounters this mysterious doctor and is influenced by him?
Of course, you have to use some cleverness to invent ways for the doctor to go about his business without being required to speak, but isn't that half the fun?