In most dramas, the boom operator will vary the distance from the microphone to the actor according to the size of shot, to ensure the production has correct "sound perspective". This simply means that when we see an actor in long shot, we expect to hear his voice as from a distance while if we see an actor in a tight close-up we expect to hear the sound in a very intimate way. Actors are often told to mime certain events, so that good sound of their voices can be taken, and the sound effects recorded as a wild track at the end of the scene, and added in during the dubbing session. The author put this as the most important because, in my experience, the main reason why good actors are not good on screen is that their vocal levels are wrong. Radio mikes pose no problem at all for vocal levels.