I find that giving people strong direction helps them relax. Anyone sitting for a portrait is worried about how they will look. Unless the subject is a pro model, they don't know how to sit, where to put their hands, etc, so they are worried about all these things. If you can make them feel like you are looking out for them, that you know what to do to make them look good, they relax a little.
That said, I agree with BrianShaw above. Some of the best moments just happen when you both have your guards down. Create the feeling that this portrait is something you are doing together.
I shoot portraits with a Bronica (similar to the RZ) as well as 4x5 and 8x10. Each has its challenges, but as the other posters have said, it's all about connecting. Large format is a pain because there is such a long lag between focussing and actually taking the shot, but I find that people love sitting in front of a big view camera, they feel like something special is going on.