Title and operating capacity - two different things. Titles are captain and first officer, operating capacity is commander or copilot. So when a captain flies not in command, he is flying in the capacity of a co-pilot. You would still call him "captain blogs" (if you wanted to be formal, although it is of course the more normal "Oi you" if being addressed by management).
Some might say "Ballast" ?!
Only dinosaurs and foolish people!
Edited to say that one of the requirments might be to be able to spell requirements!