I think you're referring to a candid expression. Some normal subjects (not models) are good at posing, or taking direction, and some are not. I think it's always a good thing to build a quick rapport with any subject you shoot, whether they're a trained model who can pull a look at will or someone who is just a regular Joe.
If you want a natural expression, they really need to be in a state of mind where they're not thinking about the shoot itself. If you talk to them in a way that makes them comfortable, but also explain that you may ask them to "hold a position", they should be able to do it well enough.
Transparency is probably the biggest factor in directing someone. If you need them to stand in the beam of a specific light, just quickly explain why and maybe a sentence or two about how it will make the photo better. If you work a specific way (slow and calculated or fast), just let them know so that they're ready.
Not to mention constant approval of the poses they are making. A lot of portrait photographers will say, "great", "fantastic", "got it, wonderful" every second or third snap just to keep the communication there and to encourage the subject.