A question about the "my aircraft" protocol led to several answers that explain how the pilot flying (PF) can delegate control to the pilot not flying (PNF).
It also seems that a pilot (e.g. returning from a break) can assert "I have control" without the other pilot first offering control.
From what I've read, unlike with ATC communications, the exact phrases used vary from airline to airline and are not the subject of rules or guidance by authorities such as the FAA. It also seems that in some airlines, some pilots are more casual about the use of standard phrases.
The only fixed idea seems to be that as long as both pilots have uttered some complementary phrase, both should now be aware that a transfer of control has occurred, and to whom.
This led me to wonder: under what circumstances can the PF refuse the PNF's request, how would they phrase this and does the other pilot or pilots always defer to the senior pilot?