Is the other guy your age, older, or younger? That's how I decide in how I answer. If he's younger, I'll answer or talk back because he might be lost (if not obviously with someone). My age, I'll either just head nod or give one word answers if I'm approached. Same with older males unless they ask for help with something.
It's rare though that I've had a guy even look at me in the rest room though let alone talk to me. It just doesn't happen that often. I keep my eyes straight forward or down and walk to my destination (stall, sink, door) without stopping.