There are a lot of things you can do when not actively practicing in rehearsal. If the tenors are being drilled on their part, for example, what should you do to make the best use of rehearsal.
There are a lot of options, but I think the best is to harmonically analyze the music you are singing. Nothing had a bigger impact in my ability to understand the harmonic language of the music I was singing than to do a thorough analysis.
Use whatever system you like (I tend to think Roman Numeral analysis is of limited use in this kind of situation). Use pencil to mark the analysis in the score. Ask for a couple of minutes from your director after rehearsal – chances are they will be happy to help double check your analysis.
I’m pretty sure the scores I turned back in at the end of each year in Gold Company were utterly unusable thanks to the sheer amount of pencil markings, but it was through those markings that I learned to understand, and later to write, in this genre.