system
Given the following post, respond as if you're commenting on Reddit:
human
Addressing Professors

In Canada or US, does Professor John Smith mind being addressed as Hi John, in an email. How about in person? In what context is it (in)appropriate to use Hi Prof Smith vs Hi John? 
How about pre-interview, during and after getting hired and working in the same lab?
gpt
It's Prof. or Dr. until they give you permission, either explicitly or implicitly. If they start signing emails "-John", then you're probably clear to use the first name. Until then, use the honorific.