This is a follow-up question to What does a head of department do?
The University of Waterloo has a document that describes the answer to this question; however the document is rather vague, and it seems to imply that if all the individual heads of departments have their departments in order (which they presumably will most of the time), the dean barely has to do anything. That's at odds with how most deans don't seem to teach/conduct research anymore however. It also seems awkward to be dean unless one is also head of department: the two people would be subordinate to each other depending on situation.