In addition to the BWRAF check, making sure you're on the same page regarding hand signals, the dive plan (purpose of dive, gas mixture, compass headings as appropriate, positive or negative entry, planned depth, bottom time, air state that means start your ascent, line vs free ascent, safety stops), and making sure each of you know the configuration of the other's gear... especially in the case of an insta-buddy.
I dive with a 7ft hose on my primary and my bright yellow octo is bungeed around my neck. If my buddy needs my reg, I want to make sure he or she knows which one to grab and which one not to. If I'm hunting, my buddy needs to understand he/she might not see as much of the reef as someone who's sightseeing because I'm stopping frequently to wrangle lobsters. If I'm on nitrox & they're on air, we might have different rates we blow through our gas (especially when the lobsters are wrangling ME). You get the point.
Be as clear as possible on as much as possible to avoid having to invent hand signals on the fly.