For news-style interviews, the lav can be clipped on quickly with the cord visible, but in fancier settings (say that PBS documentary) the lav cord can be snaked inside the subject's shirt. But--please--let the subject do this themselves! This advice will keep you out of jail.

Some types of clothing (like turtlenecks) can be a lav challenge--in a pinch you can always bring the lav around the neck from behind. Some producers prefer this technique all the time; I think it's kind of awkward. Always remember to keep the tip of the lav away from any clothing. I once hired a sound guy who--inexplicably--put the lav under the subject's tie. The resulting audio sounded like the subject was talking through a pillow--and it was a four-hour lecture!