Lack of accidents / incidents does not reflect a safe operation, not by a long shot.
Never was a more true word spoken.
Conversely, a safe operation does not mean an operation without safety incidents. What makes the organisation safe is learning from the "near miss" incidents, modifying the system and structures to remove one possibility of an accident in the future - in short, seeing possible problems and being proactive.
There are two types of pilot - those who have had an incident and those who will.
The trick consists in staying as long as possible in the first category and being aware of the dangers, of the safety margins and not eroding those margins without being conscious of them. Therefore, while Onan's friend leaned over and interfered with the power settings, it was not necessarily dangerous in itself. In interfering with Onan's attention (distracting him), in interfering with his situational awareness and in irritating him (adding stress) it
was dangerous. All this erodes safety margins. All this therefore increases the danger of the flight.
Here is where most of the problems of the subject of the thread come in.
Increased stress on the PNF (the F/O), decreased situational awareness, increased distraction all erode safety if the Captain is incapable of interfering unnecessarily. If the skipper is not a line trainer, any comments or "helpful pointers" should be saved until the post-flight debrief. Otherwise keep operational in-flight talk to SOP. Personal chit-chat is acceptable within the constraints of the Ops Manual (some say nothing except operational talk below 10,000')
Yes, the Captain is in command. Therefore he should only interfere if safety is compromised or the F/O is departing SOP without pre-briefing it. Otherwise he has no right to meddle unnecessarily and such interference should be reported by one method or another.