I usually talk to them the most when they are getting themselves into trouble. In my normal voice, as if talking to another human adult: "Dude, seriously? Why would you want to climb there?" or "What's your malfunction? I raised you to be smarter than this." or the all too common "Come on, I mean, did you really have to get your poop on the glass?" or "I just gave you fresh water, why did you feel the need to poop in it?"
I usually ask them if they're hungry before I feed them too, as if they're gonna say anything: "You hungry buddy? How's a delicious mouse sound?"... as if they have a choice.