I definitely often invoke the language of 'make them comfortable and they'll look good' -- and in general this holds true, in particular with expressions. But a lot of the time what is comfortable or natural for the couple to do, just doesn't look so great from the perspective of the camera. Most couples don't walk around with mirrors and so don't have a reportoire of poses that make them look their best. In these cases you have to have the awareness to spot this, and the facility to know how to adjust to improve things.
A few basic things that help:
-go in with at least 3-4 set poses and don't be afraid to make them happen
-keep things light and be solid enough technically (ie. not futzing with settings and light) that you can devote some attention to directing the couple, playing with them verbally.
-give the couple something to do, an activity. Even if the activity is lame (playing patty-cake, e.g.) you can get great dynamism showing even in the shots after the ostensible shot has been taken.