Please can I ask a question, perhaps of the navy types here?

I've been watching some old (1990s) and and brand new episodic SF tv these last few weeks (which by the by makes for an interesting comparison).  I won't name names because I'm not interested in either dissing them or an argument about their relative merits but if you know either you'll probably recognize them.

A couple of things came up which haven't quite rung true for me and both involve rank and/or protocol.  But it might just be my ignorance.

- are there circumstances in which  more junior officer can be made XO (Executive Officer) over someone of a higher rank?  I.e. someone who is a regular part of the same crew?

(What particularly irked me about this was that they seemed to imply that the XO was some admin function outside of the chain of command and actually had two lines about precedence in which they agreed it wouldn't affect the senior's overall seniority.  If they'd just left those two lines out I might have bought it)

- are there circumstances when, with a captain leaving the vessel for a short time, he would put a Lt in command for the duration when there's a perfectly capable (I.e. not incapacitated) commander *right there on the bridge*? (And who is the XO to boot, not some visiting dignitary)

Am I just ignorant about XOs or unusual protocol or circumstances?

I had thought the Executice Officer was basically second in  command of a ship and, in the absence of the captain, had the authority OF the captain.

In neither situation was the vessel at war although it could face combat at any moment.

tc